Hi, I’m going to introduce Leona to you and how I think you can play her the best.
I think that Leona is a great tank and if you play her the right way and know how and when to start a team fight and when to use which skill. If you know this and use it the right way you are going to win the game for u and your team, cause if a team has a great tank they win. Because most of the times the rest of the team is DPS if its AD or AP it doesn’t matter.
If they can do the damage without being disturbed, you win.
And im going to try showing you how to do this.

On the upper slopes of Mount Targon, the warriors of Rakkor live and breathe only for war. However, Targon's peak is reserved for a special group of Rakkoran who answer to a ''higher'' calling. Members of this group, called the Solari, retire their mantles of war, choosing instead to devote their lives to reverence of the sun. According to legend, the Solari were formed by a warrior who could call the raw might of the sun down upon his enemies in combat. He claimed Mount Targon's summit, the point on Valoran closest to the sun, for his solar devotion, a tradition which generations of Solari have preserved to this day. Though they maintain their reverence, no other warrior possessed the gifts of the founder -- until Leona.

Leona's parents were traditional Rakkorans, both bred for the heat of battle. To them, Leona was a problem child. She was capable of fighting as fiercely as any other -- including her childhood friend, Pantheon -- but she did not share their zeal for killing. She believed that the true worth of a soldier lay in her ability to defend and protect. When it came time for her Rite of Kor, a ceremony in which two Rakkoran teens battle to the death for the right to bear a relic-weapon, Leona refused to fight. For this, the Rakkoran leaders ordered her execution, but when they tried to strike the fatal blow, sunlight burst forth, bathing Mount Targon in light. As it faded, Leona stood unharmed and her executioners lay unconscious around her. The Solari immediately claimed Leona, demanding that her sentence be repealed. She donned the golden armor of the Solari and they bestowed upon her the sword and shield passed down from the ancient sun-warrior of legend. The Solari helped Leona focus her abilities, and when she was ready she left to join the League of Legends.

''The sun's rays reach all of Runeterra, so too must the image of its champion.''

Before I tell what runes I chose I'm going to tell you what a tank like Leona needs to function right.
1. Leona needs a lot a health, armor and magic resistance.
2. She needs to dodge several attacks for survival.

The runes I choose are:

These Runes will garantee that you'll be more protected and better as tank.

As for the previous story, the role of Leona is a full tank.
The meaning of the mastery I’ve selected is to show how much blows she can take from the enemies. The masteries will give Leona the extra bit she need to tank successfully.
That's why I’ve choses the full tank mastery.

The items I’ve chosen for Leona have a reason.
She needs a lot of health to take hits even though she has a lot of armor and magic resistance. Thats why Warmog's Armor is necessary.
The rest of the items explain them self’s i think.
If you get these items with the runes and the mastery your unstoppable.

The reason for this Skill Sequence is that she needs an escape and thats Shield of Daybreak.
If your in trouble or you see that you can get in a troublesome situation you can use it to stun and run.

Leona's next basic attack will deal additional magic damage and stun the target.

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.

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 immobilized and Leona will dash to them.

After a brief delay, Leona calls down a beam of solar energy dealing magic damage and slowing enemies. Enemies in the center of the beam are stunned instead of slowed.

The summoner spells are obvious.
The needs an escape, that’s Flash, you can use it if Shield of Daybreak is on cooldown or if you see a troublesome situation.

Teleport is for assisting your teammates, in a situation that they all go to a side lane and try to push 5 vs 2. You can teleport to assist your team and keep you turret.

Also These Summoner Spells Work:

For usefull farming use Eclips, it damages all enemy minions around you so it will be a great help in farming minions.

When you have a lot of health, magic resistance and armor you can open fights easily with Zenith Blade. You open team fights by pressing on shield of daybreak and eclipse first.
After you did that you target the highest damage dealer who try to hit with zenith blade.
If you hit the target you aimed for it will be stunned and everyone will target you but that’s no problem because you used eclipse which gives you extra armor and magic resistance for 3 seconds and after you hit a champion or several champions your shield will last 3 seconds longer. And because u used Shield of Daybreak the target will be stunned so eclipse will hit the target and your shield will last 6 seconds. Most of the times the enemy team players will try to nuke you that moment, and when they do that your team can do their damage to the targeted champions and because they are busy with the tank they have free play.