First of all, thanks for reading my support maokai guide! This guide is tested several times by myself and it always worked and made my carry fed. This is also my first guide, so I'm very sorry if it isn't very good.
"Maokai with his 'presents' for the enemy AD carry" :)

Maokai can be a very aggressive and annoying support for your enemy. He can poke with his E from a very high range and heal himself with his passive, but what makes him extraordinary is his Q and W. Every engage he can deal high damage with a snare and a slow. After hitting level 6 he can reduce damage taken in an area and absorb the damage of the enemy team. This makes maokai very good as support and very aggressive. Almost every engage will result in a kill for your carry.

Pros
+ Very agressive and good poke
+ Cc for strong engages
+ High damage on his W and Q
+ High durability

Cons
- Due poke and aggressive play you will push your lane fast
- Low mobility late game

Greater Mark of Insight - This rune is meant for extra damage on your skills. Especially your w and q. In every engage they will deal enough damage to set up an easy kill for you carry.

Greater Seal of Resilience - This rune is meant for your durability during laning face. This will make maokai even harder to kill in your engages!

Greater Glyph of Potency - This rune is meant for the extra damage, just like the Greater Mark of Insight this rune will give you enough damage to set up an easy kill for you carry.

Greater Quintessence of Swiftness - This rune is meant for your movement speed ofcourse. As your probably know, maokai isn't that fast, so with this runes you can easily catch up with the enemy carry and throw your combo on him (w-q-e)

Greater Quintessence of Potency - This rune is meant for the extra damage, just like your Mark and Glyph.

As you can see, my build is mix of tanky ap and supportive items. All this tanky ap items are for his durability and extra damage on his skills. Also the most common support item, shurelya's reverie, is very useful for maokai (mobility, catch up with enemy fleeing team or giving your team mates a speed boost to flee). Than you got the soul shroud is very useful in teamfights or during laning phase, because you can give your team/carry bonus mana regen and cooldown reduction. Also you can give yourself a little bit more durability. I think randuin's omen is very useful to slow your enemy at every possible way and it will give you lots of durability because of the armor, health and health regen. The Rod of Ages and the Frozen Heart are also for more durability in teamfights. The Rod of Ages is also good for extra damage of course!

Warding with maokai is ofcourse the same as with other support champions. Just make sure you ward your river bush with a sight ward (vision ward is good too), dragon with a vision ward and mid-late game the baron with a vision ward. You can also ward spots like the bush next to the wraiths or mid bushes mid-late game.
Play Video (an useful video made by ciderhelm)

I hope this guide will help you to be a true maokai and let you climb the ladder to succes! I know this guide isn't very long, but this is my first so I don't know everything about it. Also I really hope this was a good guide!