I typically buy Hunters Machete with 2 potions. Also, a good starting camp is Gromp. The passive he gives you is great for early ganks and also it gives you an instant level up. Make sure to have a good leash because Rek'Sai is a bit squishy at first. After you get up to 350-400 gold I recommend backing to purchase your upgraded Machete and maybe a pot or two wont hurt anyone. After Lvl3 I begin my ganks because at this point you have all your abilities apart from your ult.

I go straight to Red buff after my first back making sure that my smite is up and ready. Between each camp after that make sure to enter your grounded form because it will heal you and alert you when there is an enemy champion in your jungle allowing you to either kill or get a quick escape.I first go for Trailblazer- Devourer first. Makes for great camp clear time and gives bonus magic attack damage.

This is where her best game time comes in. She either makes it or breaks it. This is where you need to communicate with your team and determine where to gank and where to make the best plays. Also this is where you build your Ravenous Hydra and your first defensive. Whether what type of team youre playing can determine whether to build a Spirit Visage or a Randuins Omen. Both have great synergy with Rek'Sai.

Finally after you finish your Randiuns and your Spirit Visage, now you can build your final damage item, Trinity Force. Trinity doesn't really need explaining on how it works amazingly with Rek'Sai.

Hopefully this works for you as well as it works for me, If you have any better ideas feel free to comment about them. Good Luck, Challengers.