Let's start with the basics

Stat Priority

People tend to overthink secondary stats a lot when choosing gear. It's important to remember that item level is always king due to how strong main stat is. 9 times out of 10 you should be choosing your highest item level gear, regardless of what secondary stats it has. My rough rule for Leech/Avoidance/Sockets is that they're generally worth ~6 item levels. Below you'll find a breakdown of our stat priority for Mythic+ and raid.

Mythic+ Stat Priority
Haste (30-35%)
Crit Strike / Versatility
Mastery
Haste is our best and most important secondary stat as you need a base level of Haste for this spec to play well due to the cooldowns on our core rotational abilities and reliance on Holy Power generation. There are no haste breakpoints for Hpal, but higher haste means more mana strains so I recommend staying around 30-35% for season 3.
Crit lost some value due to our lessened need for hard casting in TWW, but still provides good throughput, mana efficiency and spot healing. Versatility is another great stat for Mythic+ as it increases both our throughput AND our damage reduction, which is vital for pushing keys. You'll want to get as high as you can with your these stats after reaching a solid haste level.
If you just want raw healing throughput, Mastery is your best choice, but not by a significant margin. Remember it also has no effect on our damage, which in Mythic+ is a significant part of what we do. It's a much better stat now with the return of the Beacon of the Lightbringer talent, but in general is still the weakest M+ stat.
Raid Stat Priority
Haste (30-35%)
Mastery
Crit Strike / Versatility
Haste remains one of our strongest stats in all content, taking priority over everything until you have at least 30%. Just like Mythic+, I recommend sitting somewhere in the range of 30-35% so the spec plays well but you don't strain your mana too much.
Mastery is a fantastic stat for raw healing power, especially with the return of Beacon of the Lightbringer, but it still takes a slight backseat to Haste even in raid. Use this as your stacking stat once you've hit your desired Haste levels.
Crit continues to be a strong secondary in state, even if it's slightly behind Mastery for throughput. Versatility is a solid throughput stat in raid as it will give you more healing, damage and survivability.
Manaforge Omega Tier Set

For the third season of The War Within, Manaforge Omega brings us two different tier sets each centered around one of our Hero talents. For Herald of the Sun we get the following:

2 Set: Dawnlight is increased by 60% and you gain Solar Wrath after Avenging Wrath fades, causing Sun's Avatar to linger for 10 sec.

4 Set: Solar Wrath increases the effectiveness of your Holy Power spenders by 30% and grants you 2 Dawnlight.

Rather than this tier set adding more for us to play around, it actually decreases the need for precision with timing your Holy Prisms around Wings and instead allows you to be a bit more carefree with using Prism on cooldown. It also just increases our power window of Wings and Awakening significantly, leading to fewer and shorter gaps in between our cooldowns.

Our Lightsmith tier set on the other hand, is pretty lackluster in comparison to our Herald tier set. Even though it sounds great on paper, our 4 set specifically is severely undertuned likely due to balance issues with Prot and Holy having wildly different proc rates for Hammer and Anvil:

2 Set: Hammer and Anvil's healing is increased by 15% and it can also be activated by Crusader Strike at 75% effectiveness.

4 Set: Hammer and Anvil also grants a stack of Masterwork, causing your next Holy Armament to cast one Lesser version of itself on a nearby ally for each stack.

Unfortunately unless this 4-set sees massive buffs, it likely won't even be worth equipping. The 2 set however is definitely strong and feels great for consistency of a spec that was previously a bit too RNG to be reliable.

Best Embellishments
Embellishments are a relatively small increase to your throughput, so don't stress too much over them. For START OF SEASON crafts, I usually recommend going 1h weapon -> shield, with Ascension on the weapon and Writhing Armor Band on the shield. This will give you the strongest early season boost regardless of what content you're doing, plus you can always strip the embellishments off your weapon/shield once you start crafting binding of binding or other embellishments so they aren't wasted. Below is a list of our best embellishments and which ones you should prioritize dependent on the content you're doing. For a chart of ALL embellishment values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Best for Mythic+
Your spells have a chance to proc bonus stats for allies
While this won't provide much healing, it will bring significantly more damage than any other embellishment by a large margin so it's a fantastic pickup for Mythic+. None of the more healing focused embellishments will have a significant enough impact on your healing in keys to warrant running (the good ones we use in raid drop stacks out of combat, making it bad for dungeons).
Increases Vers/Crit while above 80% health
While it won't do much for your throughput, it's better than most of the other options sadly. I generally recommend going Vers embellishment if you're going for higher push keys, and Crit if you just want to do low-mid range keys.
Best for Raid
Gain a stacking secondary stat while staying in combat
Ascension is our best option for raw throughput in raid, gaining stacks the longer you're in combat. This means that fights with heavy damage output early on won't see as much value as longer fights, but regardless of fight length this should always be your best option for throughput. I highly recommend crafting this on a weapon as your first embellishment of the season.
Doubles the effects of your Ascension embellishment
This embellishment doubles the value of your Darkmoon Sigil: Ascension embellishment, so throw this on one of your armor slots and enjoy doubling your weapon embellishment value!
Best Trinkets
Below is a list of the best trinkets you can get in the third season of The War Within, as well as the most powerful trinket combos for each content type. As it is early in the tier, make sure to check back here for updates to trinket lists if tuning happens. For a chart of ALL trinket values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Best for M+ Pugging
Drops from: Manaforge Omega (Loom'ithar)
1.5m Use: Shields 5 allies and provides them 75% DR
This is the single best trinket to ever be given to M+ healers. It won't be too good in raid but it will be absolutely game changing in keys so pick this up ASAP.
Drops from: Manaforge Omega (Loom'ithar)
Crit proc trinket with great uptime
There isn't much to talk about this, it's simply a stat stick that grants you crit when it procs, and it procs very frequently. It's our highest simming trinket for a reason!
Best for M+ Pushing
Drops from: Manaforge Omega (Loom'ithar)
1.5m Use: Shields 5 allies and provides them 75% DR
It goes without saying that a trinket this game changing in pugging will also be huge for push keys, but I'm gonna say it anyways. Get this trinket immediately.
Drops from: Manaforge Omega (Loom'ithar)
Crit proc trinket with great uptime
There isn't much to talk about this, it's simply a stat stick that grants you crit when it procs, and it procs very frequently. It's our highest simming trinket for a reason!
Best for Raid
Drops from: Manaforge Omega (Loom'ithar)
Crit proc trinket with great uptime
There isn't much to talk about this, it's simply a stat stick that grants you crit when it procs, and it procs very frequently. It's our highest simming trinket for a reason!
Drops from: Tazavesh: Gambit (So'leah)
Bond to an ally and gain their highest secondary, plus give them some
This is a great trinket in raid, but also in M+. It's a bit behind Antenna but overall it's just a simple stat stick that provides your ally a small amount of stat as well which our dps friends will love.
Other Trinket Options
Drops from: Priory (Prioress Murrpray)
2m Use: Heal a Lightshard to deal damage
This trinket synergizes extremely well with Holy Paladin in Mythic+ (THIS IS NOT A RAID TRINKET), but it can be a little awkward to get used to. The idea here is to always line up this trinket cast with your Wings, which also means you should be using your Blessing of Autumn on other players so you don't desync your wings and trinket. Use it with Wings and place it in melee (you can macro it to just drop at your feet), then spam heal it for 20 seconds until it disappears and you'll do a huge amount of AoE damage.
Drops from: Priory (Dailcry)
2m Use: Grants you secondary stats and some to allies
Signet gets stronger with how many people use it in your group, and it adds a ton of power to your wings window. More coordinated groups with 2 minute dps specs may run this for the extra stats.
Drops from: Manaforge Omega (Fractillus)
Mastery stat stick that synergizes with Voidglass weapon
Solid pickup for raid especially if you have the Voidglass Blade, but that likely won't be until later in the patch unless you're super lucky, and by then you should have access to the more meta trinkets anyways.
Drops from: Eco-Dome Al'dani (Soul-Scribe)
1.5m Use: Gain a large amount of Mastery
This is just like House of Cards last season, a simple on use trinket that provides a big boost to your Mastery. Not the greatest synergy with Hpal since our Wings are 2 minutes but it's not bad if you don't have anything better.
Enchants and Consumables

With The War Within you'll see the return of many familiar enchants, oils and armor kits as well as interesting new ones! Before every dungeon or raid boss you'll need to make sure you have everything reapplied to your armor so I suggest getting a weakaura like this as a reminder.

Item Enchants
Chance to increase your Haste
Secondary stats have become king this late in the expansion so Stormrider's Fury should now be your go to weapon enchant.
Increases your Intellect by 745
Not much to see here, main stat is king as always so don't worry about the mana enchant option.
Increase your Haste by 315 / Versatility by 390 (-110 Mastery)
Your default for raid should always be Radiant Haste (or crit depending on your gear), but in Mythic+ I recommend Cursed Versatility as you can gain more desirable stats at the cost of Mastery which has very low value in dungeons.
Increase your Leech by 1020
Leech has always been our strongest throughput tertiary stat so your default cloak enchant should be Leeching Fangs, only swapping to Avoidance in very high keys if you need it to survive one shot mechanics.
Armored Leech (Bracers)
Increase your Leech by 2040
For the same reasons you want Leeching Fangs for your cloak, you'll want Armored Leech for your bracers in most cases.
Increase your Stamina by 131
Boot enchants are never very interesting, enjoy your free Stamina!
Increases your Intellect by 747 and Stamina by 230
In most cases having the increased health pool is always better so you'll default to this but if you ever find yourself running into significant mana issues you can try Daybreak Spellthread.
Weapon Runes & Gems
Increases your Crit and Haste by 155 for 2 hours
This expansions weapon oil happens to provide us with two very strong secondary stats so make sure you stock up! This will need to be reapplied every hour. Note: If you are playing Lightsmith, you should run Rite of Sanctification instead (you unfortunately can't use both).
Increases Intellect by 181 and Movespeed per gem
As our main unique gem, it should come as no surprise we want more mobility! Reminder to equip at least one of each type of gem (sapphire, ruby, onyx and emerald) to get full value from this.
Increases Haste by 147 and Crit by 49
This is just our basic filler gem, change this out if you want more Haste/Vers/Mastery instead.
Phials & Food
Increases your Crit / Haste by 3323
Depending on which one your gear is lacking in more, you'll want to run either the Haste flask or the Crit flask.
Increases your Intellect by 445
If you have luxurious friends they may bestow upon you a grand feast, which is by far our best option for food. But for the common folk, we must dine on Jester's Board instead.
Increases your highest secondary stat by 445
This is the main personal food of The War Within, providing a large amount of secondary stat. If you don't have rich friends throwing feasts at you this will be your bread and butter.
Potions
Increases 3 Secondary Stats by 2617 for 30 seconds
Tempered Potion is the main throughput option this season but it's a bit different from previous expansions. Instead of granting main stat, it grants three secondary stats instead.
Instantly restores 4,000,000 health
As always, make sure you use your healing potions whenever you're in danger. You'll find these potions to be extremely strong self healing and off the GCD.
Restores 375,000 mana over 10 seconds (channeled)
In raid this should be your go to mana potion as it restores more than the regular one. Make sure you find a safe time and place to drink as you can't move once you start channeling!
Instantly restores 270,000 mana
This is the standard mana potion we get every expansion. Great for if you're in a pinch and need mana quickly and there's no time or safe spot to drink a Slumbering Soul Serum.
Increases up to 5 allies Intellect by 632
For my min/maxer friends, this is a unique potion that you can use right before the start of a key as your team will keep the main stat and you can use your personal pot while it's still active.
That's it for character setup, now it's time to dive into the playstyle of Holy Paladin.