AP Rek'Sai Build

General

1. You rush Hextech Rocketbelt #1, Liandry's Torment #2.
2. You get Cosmic Insight rune to use Hextech Rocketbelt more often.
3. You poke with Hextech Rocketbelt and First Strike to earn gold and burn enemies.
4. After core items, you adapt. You can play as a tank or as a damage dealer.

General

1. Use Hextech Rocketbelt active to poke your enemies all the time, earning gold with First Strike.
2. Adapt to the enemy team. Build tanky or more damage. You can play as a utility tank, poke, or even you can go some AD items to still have plenty of damage. Depends on the team and how you want to play.
3. For utility tank, remember that Liandry's Torment works good with Serpent's Fang (to counter shields) and Oblivion Orb (to counter healing).
4. Cosmic Insight reduces the cooldown of all active items, so Zhonya's Hourglass really works well with this build.
5. Wait for the First Strike and proc it with your combo, to deal damage and earn gold.
6. Be careful to not get poked, because you waste First Strike procs.
7. Your main combo is First Strike + Hextech Rocketbelt + W + Q + E + R + Ignite.
8. Your alternate combo just for poke is (if possible) tunnel E to proc Sudden Impact, then First Strike + Hextech Rocketbelt + Q.

Champion

1. You can use your Void Rush (R) for survivability and disengage. Getting focused really hard in the fight? Use it, so enemies will lose target, stop attacking you, so then you can run away.
2. You can use your Queen's Wrath / Prey Seeker (Q) (when burrowed) to scout bushes.
3. You will detect enemies moving in the bushes thanks to Burrow / Un-burrow (W) (passive part while burrowed).
4. Your ally got hit with a Snowball? Burrow and go close to them, to knock them up when they arrive. You can even time it to knock them in the air. Often it will be certain death for enemy player.

Riot

1. Keeping tunnels spread out across the map will ensure you have choices when you want to cast Void Rush.
2. Cast Burrow before travelling around the map to take advantage of the increased Move Speed and the safety provided by Tremor Sense.
3. Tremor Sense can warn you of enemies closing in, and is particularly useful when entering enemy territory.