Further readings on the Dominion Meta, which gets very few updates to actually change anything so even if these look outdated, most of it isn't

Koravel's (Beginner/Gameplay Guide) http://www.lolpro.com/guides/game-play/1434-game-play-guide-dominion/common-mistakes
Painkillar's (Tactical Guide) http://forums.na.leagueoflegends.com/board/showthread.php?t=2938089

Syl's Tier List: http://forums.na.leagueoflegends.com/board/showthread.php?t=4626720 (Aah you don't have to worship this, people can have opinions, but you get the general idea of who's weak or strong)

League of Legends Client Chat: Dominatedominion (If you want, you can come in and ask any questions like build paths on different champs, we don't have guides on all champs but theres a ton of info in there)

Rotations and Map Awareness Video (By Mirkwoodia): https://www.youtube.com/watch?v=P8HaLo-qFU0

Lastly, you can check your Dom MMR here: http://na.op.gg/
The current game MMR is sometimes inaccurate, if you hit renew data after a game, you can see your previous MMR games instead

Leona's skill is mostly about who she engages on and how she peels.

Who she engages on refers to her ability to catch primary targets out, and feed them to her teammates, waiting for them to be in position for when she stun-locks them.

How she peels is her ability to go from engage-mode from deep in a fight, to the back and start saving any damage-dealers if they can be saved. If they can't be saved, then she doesn't peel.

A quick lesson, not sure if you already know this, but will help gauge effectiveness

Evasive : Magical AoE is everywhere. It allows you to build firecape first without too much worry about MR. And I will almost always choose to build firecape as my first item.

Double-Edged Sword : Leona deals damage, not just base damage, but the Sunfire Aegis+ Odyn's Veil combo hurts.

Flash has to be used as an offensive summoner spell, to net kills and catch the back line. Using it defensively is kinda pointless because the way Dominion works is that you need to win the fight, not survive. The only exception is to survive to get back into the fight, if its going to become long enough for you to make it back.

Exhaust is always good (its meta), its an extra peel option and you can literally make someone useless for 2.5sec in a teamfight. That's a long time.

Garrison is another option for all-in team comps .

Greater Mark of Hybrid Penetration: Again, a Leona with money is a bruiser.

Greater Seal of Scaling Armor: Better than HP to reduce overall damage from enemy front line tanks, minions, towers, and synergizes with W.

Greater Glyph of Scaling Cooldown Reduction: Leona scales with CDR pretty well, but also loves non-CDR items like Sunfire Aegis and Odyn's Veil. This + Masteries means 20% CDR, but items like FotM and Locket can fill in the rest while being worth the building process.

Greater Quintessence of Magic Penetration: Helps you reach 27 Mpen, so squishies with 30-50 MR can be blown up.

Shield of Daybreak is a stun. Maxing this second over Zenith Blade means you want to be spamming stuns more, so you'll be grappling like crazy without disengage

Zenith Blade fits more into the playstyle of "Engage, lock down and tank for a bit, disengage, wait for skills to come off CD, re engage". This is my playstyle, so I max E, and at lvl 3 E to lvl 1 Q, they share the same CD. Do keep in mind Zenith is AoE and deals more damage because of that.

However if I see some hardcore engage material on my team/enemy team, I'll max Shield of Daybreak before Zenith Blade.

Hi, my name is Mirkwoodia. On normal days im stuck at around 1900 Quickfind MMR on Dominion. Max I reached was 2.2K. I pretty much started Dom so I can just play Kat whenever I want.

By the way, the max MMR nowadays is around 2.2k anyways, and I reach it whenever im on late enough.