If you feel an item, ability, rune or mastery I've chosen isn't right for your situation, don't hesitate to switch it out. This is a guide for help, not a strict set of rules to follow each and every time. Please do not down rate this guide if it doesn't work for you. Please let me know why it didn't work for you, and I'll do my best to find a way around that problem. I really do want to make this guide as good as I possibly can. I know Leona is a really new champ, and no one is QUITE sure how to play her, so I hope my little bit of effort can help everyone else to enjoy this new Champ I'm really excited to have in the game now. Good luck guys, and good tankin'!

Well, seeing as how this is my first guide I plan on putting some effort into making, I'm not going to have the full guide written out for a while. I hope to flesh it out more and more as I become familiar with Leona, but for now, this is what works for me.

Pros:
Great at initiating a fight.
Blink counters tanks standing in front of squishies
Space Laser ulti
Armor and MR boost makes for surprisingly tough kill
FEMALE TANK
Cons:
Dependant upon teammates for fights.
Not good at running away
Can sometimes hit wrong target with Blink

Leona's damaging spells affect the target with Sunlight for 3.5 seconds. When allied champions deal damage to those targets they consume the Sunlight debuff to deal 20 / 35 / 50 / 65 / 80 / 95 / 110 / 125 / 140 additional magic damage. Leona is unable to activate the additional damage herself.

Leona's next basic attack will deal additional magic damage and stun the target for 1.25 seconds
Cost: 45 / 50 / 55 / 60 / 65 mana
Cooldown: 12 / 11 / 10 / 9 / 8 seconds
Magic Damage: 40 / 70 / 100 / 130 / 160 (+0.3 per ability power)

Leona raises her shield to gain bonus Armor and Magic Resistance for 3 seconds. When the effect ends she deals magic damage to nearby enemies and prolongs the effect for a bonus 3 seconds if any enemies are struck.
Cost: 60 mana
Cooldown: 14 seconds
Armor & Magic Resist Bonus: 30 / 40 / 50 / 60 / 70
Magic Damage: 60 / 110 / 160 / 210 / 260 (+0.4 per ability power)

Leona projects a solar image of her sword, which deals magic damage to all enemies in a line. When the image fades, the last enemy champion struck will be briefly immobilized and Leona will dash to them.
Range: 700
Cost: 60 / 65 / 70 / 75 / 80 mana
Cooldown: 13 / 12 / 11 / 10 / 9 seconds
Magic Damage: 60 / 100 / 140 / 180 / 220 (+0.4 per ability power)

After a brief delay, Leona calls down a beam of solar energy dealing damage and slowing enemies by 80% for 1.5 seconds. Enemies in the center of the beam are stunned instead of slowed.
Range to Center of AoE: 1200
Cost: 100 / 150 / 200 mana
Cooldown: 105 / 90 / 75 seconds
Magic Damage: 150 / 250 / 350 (+0.8 per ability power)

I chose to skip the magic pen in the first tree in favor of the ghost mastery, because your main focus shouldn't be damage. Your job is to get in there and stun the right people, as often as you can, as effectively as you can. Don't forget that. You're a tank/support before anything else. Keep your team alive, and fed, and you'll be sure to win every game.

[Greater Mark of Insight]
There aren't really any good tanking marks, so the best way to use these slots is to give a slight boost to your abilities. Not sure if this applies to sunlight damage or not yet.

The early game boost in your armor is really what's gonna carry you into your mid game while you build up your [Soul Shroud]

With these, you can reach your full 40 CDR without Blue buff or a third cooldown item, opening you up to tant items as well as leaving Blue for someone on your team who could better use it.

This is the only viable way of giving Magic Resist before you hit your mid game, so I decided to go with it. If you think you could use some HP, you could always vie for That one seems to work pretty well also.

My personal favorites for Leona are and , for the dual purpose of killing and escaping they both provide.
If you prefer to use something else, I'd recommend for the map presence it grants you, for the same reasons as ghost and to get you in reach for that NEEDED stun or to have an unbreakable laning game.
Honestly, with Summoner Spells you can't really go wrong (except with smite). They all have their uses for Leona! (EXCEPT SMITE)

This skill is Leona's bread and butter. You wanna focus this skill first to maximize your lane game with your partner, netting them kills, or yourself even. Anytime you can, make sure you use this skill AFTER your auto-attack, to maximize your damage.
Once you hit your full 40% CDR, you'll be using this once every 5 seconds or so to dominate team fights.

THIS is what makes Leona as beefy as she needs to be to do her job right. Once you've got this maximized, you'll be taking damage with the best of them even if your build is a bit behind. You're gonna wanna focus this skill second.

For the majority of the game, you're only going to need 1 point into this skill, so you're going to max it last. The main function of this skill is initiation. Use this to get right in the middle of the fray, and start doing your thing by stunning everybody.

This ulti is probably one of the most fun I've used in the game. Not only does it have an immense range (nearly a full screen), it comes with a decent chunk of damage, a whopping 80% slow, and a multi-target stun at the center. Not to mention, it becomes all the more fun when you imagine

Honestly, tanking is more about playing it by feel from game to game, so feel free to mix it up a bit, but here are some of my suggestions. If you find that your enemy team is stacking heavily on one kind of Damage, it would be best to invest more in the appropriate armor to counter them.
If your opponents choose to rely heavily upon Physical Damage, you may want to consider buying a in place of your Abyssal Scepter.
Should your enemies choose to go for more of a Magic Damage team, you could substitute a for your Randuin's.
If you find that you just can't keep yourself alive during your team fights, it might not be a bad idea to get a for the dual armor's and revive.

Team Work is KEY for Leona. Without a good team, Leona gets shut down easy. You need to have a heavy burst team that can take your opponents one at a time. You have to get in there and stun their carry, then laugh as they die in a second. Wash, rinse, repeat.
During your laning phase, you have to, have to, HAVE TO communicate with your partner. You need to be the one starting the fight, and keeping them alive long enough to kill your enemy. Don't rush in head first and cry when you die alone because your partner didn't know to go in with you.
If for even one second you lose sight of your lane enemies, IMMEDIATELY cal that MIA. It will save so many teammates if you learn to do this right.

Just threw this build together after a suggestion. I'm gonna playtest this myself, and put down my results as soon as I can. If anyone else wants to give it a shot, please do, and give me any feedback to tweak it, or dismiss it.