It is my first guide, and my english isn't the best soo... please dont be too much critical.
I have hope this guide will help you in playing Maokai. :)

Ignite - In early game it is useful, when you nuke enemy and he start running away you can kill him easly. If you play longer use ignite on start and use skills after that (when ignite is on coldown you gain 5 ap, it is'nt many but ever it is).
Flash - When you use Twisted Advance and suddenly situantion becomes dangerous you can ever escape from fight using flash.
Additionaly whe you chasing enemy you can instantly cath him using Flash and Twisted Advance.

Sapling Toss - It is your main skill, us it to maek enemie's hp lower, this skill starts fight.
Twisted Advance- You us it secondly. If you want make gank from ambush use it first to cath enemy.
Arcane Smash - Useful cc, after using Twisted Advance go behind enemy wait when roots end and hit enemy to puch him some and gain additional fight time.
Vengeful Maelstrom - Us this ability thats fast as you can, active "ka-bum" when enemy have low hp, it is yseful to gain kills, support team in temfights, and making area damage.