Hello the all! This is my first guide that I have written on here.
I don't claim to be the best, nor do I assume it.
This may not be the best guide out there, nor do I claim that to be true, but this is how I play Leona, and I have done fairly well with her thus far.

This guide is focused on building Leona as a tank, following her original design plan. I have messed around with AP Leona, and it isn't nearly as fun as being a constant pain in the enemies butt.

Let's start off with my rune selection.

Greater Mark of Magic Penetration x9
Greater Quintessence of Magic Penetration x3
Greater Seal of Evasion x9 or Greater Seal of Armor x9
Greater Seal of Magic Resist x9

Magic Penetration, is simply amazing on Leona. It makes all of her abilites that much harder, and allows her Sunfire Aegis to do a little bit more damage, since she's going to be in the middle of the battle, trying to control the fight as much as possible.
You could switch out the Dodge runes, for the Flat Armor runes, but I feel that the Dodge helps her out more late game, while the Flat Armor buffs her early game.

0/21/9
Is a fairly common mastery page set up for tanks. You can swap points around for personal preference, but I have found that these work best for my play style.
Strength of Spirit is nice to have, because it gives your fairly decent mana pool at end game, adding a little extra HP/5 that could very well save your life.
Everything else is pretty self explanatory, seeing as the mastery's synergize well with her role as a tank.

Regrowth Pendant + Health Potion In my opinion, are the most comfortable starting items in the game, for a good majority of champions.
Grab a Philosopher's Stone to make harassment a bit easier to deal with, and having to worry less about last hitting.
Boots should be grabbed next, no explanation necessary here.
Heart of Gold acquired when you reach 825 gold.
Mercury's Treads should be completed next, as they will help you soak up a good portion of enemy CC. (95% of all CC does Magic Damage, by the way)
At this point you start building against what will be doing the most damage to you.


Heavy AD enemy comp?


Grab Randuin's Omen, building Warden's Mail first.
(This will give you an awesome AoE slow for chasing, and for running away.)

Building a Force of Nature next, to give you a slight boost to MS, and making you even
more unkillable with the amazing passive.

Next grabbing a Frozen Heart for it's armor, and much needed CD reduction.

Finishing off with Sunfire Aegis and Shurelya's Battlesong.

By this point, with a good team, and in my experience, you should be an unkillable machine.

Tons of Magic damage in enemy comp?

The only difference between here, and the above paragraph, is that you build your Force of Nature first, building Randuin's Omen next. Maintaining build order with, Frozen Heart, Sunfire Aegis, and Shurelya's Battlesong always being built last.

Balanced enemy comp?
Grab Chain Vest and a Negatron Cloak, building your Sunfire Aegis first, then building against which ever enemy champion is doing the best/most damage at the moment.

Alternatives
I find that Shurelya's Battlesong is underused at the moment, but I like to get it as my last item, because I like using it's active for quick tower pushes. This item is replaceable, and don't think that it is necessary to have, to be the best Leona ever.

AD Carry wrecking your face?

Get

Tired of that first CC?

Get

Dying first getting on your nerves?

Get

Randuin's Omen on CD more often than you get into team fights??

Get or at this point, it all comes down to personal preference.

You want to level Shield of Daybreak and Eclipse fairly evenly.
While putting one point into Zenith Blade at level three, and not touching it again, till level 12 & 13 to even out the damage with you Q&W abilities.
This allows your stun, and your Armor/MR steroid, to be on a low CD, allowing you to mitigate more damage, and disrupt fights easily at low levels.
Level your ultimate whenever you can, as it is easily my favorite tank ult in the game. It's plethora of uses is amazing, and I absolutely love killing someone with it as they start running away.

Let's see.