Jungling with Dorans Blade can be a serious challenge, and one little screw up and you can die to the jungle. However, if you get it right you will be set to rampage with Rengar! To start off, I personally go blue buff first, but you could also go with what ever side will give you a better leash. Upon getting your buff you MUST have 4 ferocity stacks or even 5 if you can. After the leash you need to have at least 250hp left, but the more the better. Then head over to red buff (or blue) and prepare to fight, this part is important to understand so you dont die 3 minutes into the game. If you are fighting red buff make sure to go into the bush right next to it, this makes sure there is nobody there trying to counter jungle. Leap onto lizard and Q to get 5 ferocity. NOW INSTEAD OF HEALING WITH W, hit with Q again to insure a quick kill. Be sure you smite appropriately assuring no buff steal AND so there is no unnecessary missing hp. Now head over to wraiths and wolves using your 5 ferocity stacks to Q. If you are really low on hp and I mean REALLY low then W with ferocity but otherwise use Q. Now that you have completed the wolves recall and buy A SECOND DORANS BLADE. This insures insane burst damage. Congratz you have officially completed the hardest part of dorans jungling. At this point I usually look for top or bot ganks and for counter-jungling. It's much harder to gank mid considering the enemy is usually out of range of your passive leap. Renagars ganks are always insane and pretty hard to not at least make the enemy back. The most important thing to know about ganking with rengar is to stay in the bushes. If an enemy is outrunning you, run into the bush right next to you and get the kill :D.

Pros

Manaless
Point target crowd control
Free tank stats from Battle Roar (W)
Extremely powerful in jungle skirmishes
High single target burst with ferocity
Incredible offensive steroids

Cons

Rengar is subpar without Ferorcity, and has difficulties generating it early levels
Farm/kill dependant