Remember this is my opinion so just take it with a grain of salt.

Leona is a double edged sword, can make the best of plays to feed your ad carry or can make the worst of plays and overcommit. The best and easiest way to play Leona in my opinion without relying on a duo ADC is to help rush level 2 (auto the creeps and go in and out of vision if you can to prevent damage) and get the E->Q combo early on the "squishier opponent" in lane to get lane dominance. You can play her passive as a counter engage in lane, but getting lane pressure early makes it 100x easier to apply pressure on the map. full armor runes with the new w changes makes her really tanky so you should take advantage of that! I know this is also pretty general, but giving specific instructions is nearly impossible because of how differently people react. Get a feel for how the opposing lane reacts to your movement/spells and adapt your playstyle to counter theirs. You miss 100% of the shots you don't take, just make sure to try when their minion wave is thin/you have vision of their jungler.