dblade stacking sucks now because they nerfed it. don't do it.
rengar takes a lot of experience and matchup knowledge to play correctly. some champions you can outright nonstop just on at level 1 and beat them, and others you have little to no chance against.
takes a lot of rengar top experience to know the matchups well. generally you can outtrade a lot of champions with empowered Q. some players take ignite, others take exhaust. i prefer exhaust. at level 6, you can just gank bot or mid for pretty easy kills. just get 5 fero before you ult, and make sure ur laner doesnt make it too obvious. hopefully you know how to empowered E during leap. this makes rengar ganks pretty much guaranteed kills.
u probably want to rush tiamat for the pushing power and damage it gives to ur burst. u should get ionian's so your ult is off cd faster.
there is so much more about rengar but this is the gist of it. if you want to know how to play rengar better or get better knowledge on the lane matchups, just watch a dekar or ryan choi videos. i'd recommend dekar, though, because ryan choi took a break and a lot of vods of him are almost a year old, which is when he still stacked dblades. dekar's channel is here: https://www.youtube.com/channel/UCj1NEvOGuhf9UoknPBF3pbg/feed