This guide is just a quick look at Leona and how I personally play her.
Just remember Leona is a tank with many stuns and damage reduction she is meant to charge into the enemy cc and watch people die.

The items above are NOT SET IN STONE. pick up a Thornnmail before force of nature if the opposing team is heavily AD based.

This will probably be different for everyone and changes with the situation but for the most damage use Eclipse then Zenith Blade then Shield of daybreak and save Solar Flair for runners or if you need a little extra damage.

Teleport, Flash, and Surge are also useful but i have found the two up top the best for how i play.

My reason behind the runes is you will dominate early game. If you get your lane partner fed you have done your job the extra damage will help a ton in bringing enemies down.

Leona is hard to farm on which is my reason for grabbing Sunfire cape early on. It helps you get lots of last hits you normally wouldn't.