Wghen you gank dont be too aggro, you are wanting to use your e to get behind them running or them fighting, you want to dodge as muuch as you can because HP wont last forever. so let the laner tank a little then go in ha when the enemy is half HP and steal the kill or take the assist depending on what you want to do.

Tips:-
use your E in the biggest part of a wall, it will make it longer.
use your W twice on buffs the stun makes it EZPZ to kill.
Start at Blue, i know you dont need blue no mana, right? WRONG!!!!! it is gold, it is EXP it is a blue they will look to steal because you dont need it. so yes go blue first.

With rek'sai you are early game low dmg low tank. so we are looking to farm out jungle while poking after lvl 4. only poke don't go in for fights unless you are 3v2/1.
Rek'sai lets you counter jung very easily. so you arte wnating to take their buffs and jung monsters while they are low lvl ganking. if they dint gank low lvl just creep into the enemy jung and stall all you can the back and go to yours. you need gold as much as you can get. after cinderhulk and hydra are complete you may gank.
!ENJOY!

Feel free to take flash off for ghost or TP, TP gives you map control and ghost is an easy flash if you flash into walls.