Hi there,
I'm no expert on League of Legends, in fact I'm Silver (I know I suck so.. stop complaining), but I decided to make this Build Guide to show you guys out there my "Fun Build" and also my "Serious Build".

What are these builds?
So these builds are for my favourite jungler Rek'Sai. Rek'Sai is a great jungler when it comes to clearing camps, if you're careful enough you might be able to clear them all before your first back.

NOTE:
This "Tutorial" is my opinion so do not expect it all to be correct and a Challanger level Turotial.

Thanks.

He is an amazing jungler early game with his great ganks (After his first back he is good to gank as he already has his tunnels ready to start diving in on the laners) late game because of his kit being so good at focusing one person and "assassinating them" (ADC's). You basically just DIVE IN knock the ADC up with your Un-Burrow (W) start "SPINNING" with your Q "BITE them" with your E and get out.

This may not be accurate!

Ok here I'll "explain" a little bit what kind of options you have when it comes to jungle items.

The Stalker's Blade is a GREAT buy for Rek'Sai as you have another form of CC rather than just using your Unburrow it is extremely helpful for ganks and to help you stick to your opponents.

The Skirmisher's Sabre isn't that great of an item on Rek'Sai but it really makes her deal TONS of TRUE DAMAGE from both her attack speed (Which you have if you use this build) and her Furious Bite

The Warrior enchantment is amazing on Rek'Sai as it gives you AD for faster camp clearing and makes you deal more damage to laners when you decide to go for a gank.

The Devourer enchantment is "good-ish" on Rek'Sai as it gives you more attack speed (That way you gain more Fury of the Xer'Sai (This enchantment is not recommended, you should only make this enchantment for "Fun" or when you feel like you actually need it). Also as of the new patch 5.13, this enchantment now stacks on kills, assists, and camp clearing. As you may already know you now have a wolf companion that grows with stacks and at it's full stack capacity you become the "Devourer" and gain another UNIQUE Passive which makes you deal a "Ghost attack" on every other attack which activates all on-hit effects twice (Which is good on this build because of the Life Steal and the Ravenous Hydra.

The jungle route:

You should first start on the Krug (Krugs) [Of course, Rek'Sai doesn't need mana... DUH] then clear the Red Brambleback (Red Buff), move on to the Raptor (Raptors) clear them, continue on and clear the Blue Sentinel (Blue Buff) and then the Wolf . (You might not be able to do this route on one go or at least you won't be able to actually kill the Gromp )
TIP: If you start on the blue side of the map (This also works on the red side but two people are better than one) you can ask for a Hard Leash on the Krug so that way you save the smite for the Red Brambleback and heal yourself with it.

You should now back and buy your items.