I leveled a Smite Priest using this build: https://wotlk.evowow.com/?talent#bVcruhhiRIo0bZcxxhbbVbZh:ubiMzc
Similar to your build but mine is different. As you can see, I did not take Darkness nor did I take Improved SW:Pain. In fact, my rotation did not involve the use of SW:Pain unless I was solo'd an elite. The same rule applied for Devouring Plague -- I used it only on an elite.
Holy Fire > Smite > SW:Death to finish off the mob when it was at low HP. The talent, Spirit Tap, helped tremendously with the mana. This is the guide that I followed.
The Path of Light: 1 - 80 Smite Leveling Guide [3.3]
A priest's offensive arsenal is made up of two school of spells: Holy and Shadow. This guide explores the talent build followed by those who wishes to level using the Holy school of spells. It focuses primarily on talents in the Discipline and Holy tree.
Before we get started, here are some helpful tips
Always make sure you have the most updated wand possible. Wanding costs no mana, so it's great for finishing a mob off when it's at 10% health.
Level first aid as you go. Bandages cost no mana and the profession is great for getting rid of those cloth scraps stuffed in your bags.
Save up gold for glyphs.
Carry a few stacks of food and water, a few mana and health potions for emergency.
When you see another class on the road, cast Power Word: Fortitude and Divine Spirit on them. They will most likely return the favour and give you Mark of the Wild, Arcane Brilliance or Blessing of Wisdom. These short buffs will improve your stats temporarily, making you level faster at least for a little while.
Make friends, party up, drop a heal on someone in danger. You are a priest, play as one.
The build we are aiming for looks like this: 41/27/3 Holy DPS Farming Build Spell Rotation
Level 1 to 3: Smite until dead
Smite is your only offensive spell besides melee, there aren't a lot of options here.
Level 4 to 9: Smite -> SW:Pain -> Smite until dead
For in depth breakdown, please visit my Leveling Oom post
Level 10 to 19: Mind Blast -> SW:Pain -> PW: Shield -> Smite until 5% -> Wand until dead
Mind Blast does a lot more damage than Smite at this point because of rank differences. It also takes a lot less time to cast. Use this as an opener so that it will be available again for the next mob.
Level 20 to 61: PW: Shield -> Holy Fire -> Smite x 3 -> Mind Blast or Wand
You will learn Holy Fire at level 20. From now on, it is important to pre-shield to avoid interruptions when Glyph of Smite is in effect. Wand until Holy Fire cooldown is finished, then move onto the next mob.
Level 62 to 80: PW: Shield -> Holy Fire -> Smite x 3 -> SW:Death
Use Power Infusion and Inner Focus every cooldown. Don't forget to cast PW:Shield because you now have the reflective shield talent.
Fighting Elites: SW:Pain -> Devouring Plague -> PW:Shield -> Holy Fire -> Smite x3 -> Mind Blast.
Keep DoTs up, refresh shield. Rinse and Repeat. Shadow
Level 10: 1/3 Spirit Tap
Level 11: 2/3 Spirit Tap
Level 12: 3/3 Spirit Tap Maxed Out! Holy
Level 13: 1/5 Holy Specialization
Level 14: 2/5 Holy Specialization
Level 15: 3/5 Holy Specialization
Major Glyph I: Glyph of Inner Fire or Glyph of Shadow Word: Pain or Glyph of Power Word: Shield.
At level 15, you will unlock your first major glyph slot. Since Smite damage at early levels are not that high until you reach Searing Light, you will need to cast Shadow Word: Pain as part of your rotation. Glyph of Shadow Word: Pain will give you some mana back. However, depending on your server's economy, this glyph might be priced higher than usual depending on player demand, so an alternative is to grab Glyph of Inner Fire instead.
Glyph of Inner Fire will reduce the physical damage taken by your priest by approximately 8%. Less damage taken means higher survivability and less downtime spent on casting healing spells. Edit: A third alternative is to take Glyph of Power Word: Shield. The 20% healing from PW: Shield can save you a cast of healing spells here and there, thus saving mana and helping you survive better. It should also be noted that Glyph of PW: Shield can proc Surge of Light at later levels! (Courtesy of Luthvian)
Level 16: 4/5 Holy Specialization
Level 17: 5/5 Holy Specialization Maxed Out!
Level 18: 1/5 Divine Fury
Level 19: 2/5 Divine Fury
Level 20: 3/5 Divine Fury
Major Glyph I: Replace with Glyph of Smite
At level 20, you will learn a new spell called Holy Fire. Glyph of Smite will now come into effect so purchase it as soon as possible from your closest Auction House. Unfortunately, you will not get another major glyph slot until level 30 so you must destroy whatever glyph you have in your slot for now to make space for the Smite glyph.
Level 21: 4/5 Divine Fury
Level 22: 5/5 Divine Fury Maxed Out!
Level 23: 1/1 Desperate Prayer
Level 24: 1/3 Blessed Recovery
Level 25: 2/3 Blessed Recovery
Level 26: 3/3 Blessed Recovery Maxed Out!
Blessed Recovery doesn't heal for all that much but the keyword here is that it doesn't cost any mana at all. It works regardless of whether the damage you take is absorbed or not. So a small ticks here and there might just save a few casts of heals from time to time.
An alternative to Blessed Recovery is to take 3/3 in Improved Renew. Renew is instant and can be used on the run or in between pulls to fill your health up. It might be a slow heal but when you are in no danger of dying, it is your most efficient heal.
Level 27: 1/2 Healing Focus
Level 28: 1/2 Searing Light
Level 29: 2/2 Searing Light Maxed Out!
Level 30: 1/2 Holy Reach
Major Glyph II: Glyph of Holy Nova or Glyph of Inner Fire or Glyph of Power Word: Shield
At level 30, you unlock your second major glyph slot. If you are feeling a bit daredevil-ish, grab Glyph of Holy Nova and try out some aoe grinding. Cast SW:Pain and Devoured Plague on the first mob, and SW:Pain on all other consecutive mob you pull. PW: Shield yourself, and wait until all the mobs are gathered neatly around you. Holy Nova spam until everything is dead. Cast a Renew on yourself to even out some of the damage. If you accidentally pulled too many, drop a fear bomb (Psychic Scream) and heal up. Make sure you are at full health and mana before every pull. Don't try pulling too many at a time or else you will certainly die. Five is usually a nice number, but you can pull more if you think your mana pool can sustain it.
If you prefer single target steady leveling, grab Glyph of Inner Fire or Glyph of Power Word: Shield.
Level 31: 2/2 Holy Reach Maxed Out!
Level 32: 2/2 Healing Focus Maxed Out!
Level 33: 1/5 Spiritual Guidance
Level 34: 2/5 Spiritual Guidance
Level 35: 3/5 Spiritual Guidance
Level 36: 4/5 Spiritual Guidance
Level 37: 5/5 Spiritual Guidance Maxed Out!
Level 38: 1/2 Surge of Light
Level 39: 2/2 Surge of Light Maxed Out! Discipline
Level 40: 1/5 Twin Disciplines
Level 41: 2/5 Twin Disciplines
Level 42: 3/5 Twin Disciplines
Level 43: 4/5 Twin Disciplines
Level 44: 5/5 Twin Disciplines Maxed Out!
Level 45: 1/3 Improved Inner Fire
Level 46: 2/3 Improved Inner Fire
Level 47: 3/3 Improved Inner Fire Maxed Out!
Level 48: 1/2 Improved Power Word: Fortitude
Level 49: 2/2 Improved Power Word: Fortitude Maxed Out!
Level 50: 1/1 Inner Focus
Level 51: 1/3 Meditation
Level 52: 2/3 Meditation
Level 53: 3/3 Meditation Maxed Out!
Level 54: 1/3 Improved Power Word: Shield
Level 55: 2/3 Improved Power Word: Shield
Level 56: 3/3 Improved Power Word: Shield Maxed Out!
Level 57: 1/3 Mental Agility
Level 58: 2/3 Mental Agility
Level 59: 3/3 Mental Agility Maxed Out!
Level 60: 1/1 Soul Warding
Level 61: 1/2 Reflective Shield
Level 62: 2/2 Reflective Shield Maxed Out!
At level 62, you learn a new spell called Shadow Word: Death. This spell makes a nice finishing move and it is instant. If you like using this spell, replace Glyph of Holy Nova with Glyph of Shadow Word: Death. This will increase the damage done by SW:D by 10% for mobs under 35% health.
Level 63: 1/5 Mental Strength
Level 64: 2/5 Mental Strength
Level 65: 1/2 Focused Power
Level 66: 2/2 Focused Power Maxed Out
Level 67: 3/5 Mental Strength
Level 68: 4/5 Mental Strength
Level 69: 5/5 Mental Strength Maxed Out!
Level 70: 1/1 Power Infusion
Congratulations on reaching level 70. From here on out, you have two options. If you wish to continue the path of solo leveling, follow this guide. If you are more interested in healing instances, try reading my other 70 to 80 wrath leveling guide which further breaks down the talents and explains some of logic behind it.
Level 71: 1/3 Focused Will
Level 72: 2/3 Focused Will
Level 73: 3/3 Focused Will Maxed Out!
The reason I'm recommending Focused Will before anything else here is to give you the option to start some serious aoe leveling via Holy Nova. Focused Will not only gives you extra crit, but also significantly reduce the amount of physical damage taken when you are beating on by a train of mobs.
As for pulling style, the generic is quite simple. PW:Shield yourself, gather up the mobs, activate Power Infusion and do Holy Nova spam until everything is dead. Use the Surge of Light proc to kill off the lowest health mob to proc Spirit Tap. Also, use Surge of Light to heal yourself if your health drops too low.
An alternative is the following:
Grind Holy Fire/Smite style until 25% mana (but 100% health)
Send out Shadowfiend (Offensive) on target A, activate Shadowcrawl.
PW: Shield yourself and pull target B,C,D,E,F,G etc.(as many as you can handle, but no more than 10) do this quickly.
Gather B,C,D,E,F,G neatly around you and run to Target A.
Shadowfiend will now expire, you should now have 90% mana. Target A should be at near death.
Activate Power Infusion and start casting Holy Nova.
The first few casts will kill Target A, its death will grant you Spirit Tap.
Use Surge of Light Smite to kill off fleeing mobs or use Surge of Light Flash Heal to heal yourself
Spirit Tap from target A will now expire, target B,C,D,E,F,G etc will drop dead soon after, and a second Spirit Tap will kick in while you loot the bodies
Sounds complicated? Perhaps, but the whole point of this is to make sure you have Spirit Tap up for the whole duration of your Holy Nova aoe, as it will boost your in combat mana regeneration significantly.
Shadowfiend has a long 5 minutes cooldown, so you need to make sure that you take full advantage of it when it's up. For this reason, it is not recommended that you use it on the same target you are smiting, as the target will likely die before your Shadowfiend expires.
Level 74: 1/3 Enlightenment
Level 75: 1/3 Rapture
Rank 1 Rapture will essential reduce the mana cost of your PW:Shield by 50%
Level 76: 1/2 Aspiration
Level 77: 2/2 Aspiration Maxed Out!
Level 78: 2/3 Enlightenment
Level 79: 3/3 Enlightenment Maxed Out!
Level 80: 1/1 Pain Suppression
At level 80, you will unlock your final major glyph slot.
Congratulations on reaching level 80. I hope you have enjoyed Smite leveling so far, but the fun has just started.
The 41/27/3 build you now have at level 80 is actually a farming build designed specifically for doing daily quests. So use it as a dual-spec option for saving up that epic flyer or other things required for the many gold sinks Blizzard have made for us.
Major Glyphs: Glyph of Smite, Glyph of Holy Nova, and Glyph of Shadow Word: Death (single target heavy)/Glyph of Inner Fire (aoe heavy)
Minor Glyphs: Glyph of Levitate (for fun). The other two slots are completely up to you.