I'm not the top 1 rek'sai in the world not even a good player, but I really like this champion, so I wanted share my way of playing her and my thoughts.

I am not a native, so forgive me if I made grammar errors.

* Rek'Sai is a strong early and mid game champion, so it has a lot of snow ball potential.
* Her ult can dodge a lot of damage and is good for finishing your prey.
* She can fall behind at the late game, but with a good start this won't matter that much.
* With this build Rek'Sai becomes a squishy champion and that get melted really easily, but she gets a lot of burst damage.
* The buried form can be used to locate champions at places with no vision and invisible champions get caught by "true vision", some times it can get a bit annoying.
* Unique pathing options with her tunnels and moderate easy scapes.

* Not good at 5vs5 teamfights, however she excels at small quarrels and ambushes.
* When buried you can't tell enemies apart.

Rek'Sai is a really strong early game champion, so early ganks are her speciality.
Always look in the map for gank opportunities, being active in the game is a priority since she wont get as strong just by farming. Don't gank when you felling insecure, sometimes its better to gank a lane that already have a advantage than ganking a lane that is behind just to try to make the game even, because if something goes wrong there is the possibility that everyone dies.
Being active in the game does not only means to just gank a lot and getting a lot of kills, its also very important to do the objectives.
Don't forget to set wards as you play.

Rek'Sai is not really good at 5vs5 team fights, so try to get someone that is out of the group or alone.
When you are going to gank the bot lane try to hit a perfect gank, since rek'sai gets really vulnerable when she don't hit her initial charge. You can counter that a bit by building Galeforce instead of Kraken or Shieldbow.
Try to do a split push instead of teamfighting all the time, especially when your team is playing well.

You are basically doing this:
When buried: first thing hit the buried Q - W - AA (use your Q to cancel your auto atk)- Q hit three times - E.
You are going use your R as a finisher or to dodge an dangerous abilitie and rarely to engage.
When using Galeforce is like this: buried Q - Galeforce - AA (use your Q to cancel your auto atk)- Q hit three times - E.

So the guide is ending...Well anyways, Rek'Sai is a bully on the early game when played properly, so be agressive, but don't get too full of yourself as the game goes on and don't focus too much in getting kills. Remember that you are quite squishy, so play smartly.
This is it, I hope it helped you, if you want more details try checking LordGrox Rek'Sai guide for a complete guide on how to play rek'sai.
Crit'Sai is a off meta build so be carefull.