Starting

Now, the build seems basic I understand, but, as Rek'Sai you have the ability to gank lanes at level 3. Here is how I tend to play:

Entering the game, seeing as Rek'Sai needs no mana, I go to red buff. I get my Q, Queens Wrath, for the aura damage against the creeps. After getting red buff, get your W, Burrow. This allows you to burrow and gain health back on your way to getting wraiths. Start with your Q whilst borrowed, Prey Seeker, then proceed with knocking up the wraiths and continuing your Q. Burrow again to Wolves to again get health back and then, after getting the wolves, gank mid lane (if needed) It is straight forward form there.

Ganking

Assuming you are not level 3, ganking is easy. To begin, you want to burrow and stay hidden; her tunnel whilst borrowed gives amazing ganks! So, stay behind a wall then use your tunnel to get under it, once out, shoot with her Prey seeker then go straight to the target. Unborrowing underneath the target will stun the champion, allowing you to quickly use your Q for quick slices and then, once Rek'Sai's fury is at maximum, use her E, furious bite, to finish the target. If the target is getting away, simply burrow again, if your tunnel is ready, use your tunnel to get closer but if not, use her Prey Seeker to see if you can finish the champion off before getting to safety.

Runes

With my rune page I tend to go for damage and health. This allows for higher sustain whilst in team fights and to tank damage whilst ganking lanes.

If you can improve the runes, go ahead, these are just my preferred runes as I prefer to sustain damage.

Don't bother with the skin, just a tip.