Welcome to my Rengar guide.

I've been playing Rengar almost all of my games for the last year. I've faced every single matchup available in the game, I've played Rengar every role and shape, trust me, top lane is the best one.

Take control of the brushes!

Beginning
At 1:56 the 1st minions walk by your enemy's turret. Hit a Q on the 1st minion in the wave then make the minions chase you towards the brushes. Then proceed to walk into the brushes and last hit with your passive. When ever you got your Q ability available, use it to lasthit while you use your passive. Stack ferocity up. When you got 5 stacks of ferocity (full) proceed to jump on your enemy and poke them with your double Q, trust me, the amount of damage output will surely surprise them. Stay offensive at all costs, try for the first blood or the first kill of the lane.

Middle
Now you should have enough items to deal massive damage (your doran's blades). Try to stack your ferocity up and go for kills against your enemy. Try pushing out your lane as fast as possible and roam mid anytime you got your ultimate available.

End of laning phase
The enemy turret is now down and you're free to roam. Stack your ferocity up through jungle camps or pushed back lanes. Keep in mind: always have your ferocity at full stacks, don't walk with empty ferocity bar or half stacked, it should always be full!

Goal
Your main goal is to assassinate splitpushers or enemy players walking in the wrong jungle or walking without the team. Usually the ad carris are companied by their support player and the mid laners are usually companied by the enemy jungler. You should through mid-game have a heavy damage spike which allows you to almost assassinate any player in the enemy team.

Don't be front-line
Never stay in the front-line! If it's possible, never group unless you prepare for objectives or are trying to force a fight. Stay either at top or bot lane trying to splitpush and gain solo exp from minions, because level advantages scales really well with Rengar since you want to get maximum level on your ultimate ability as fast as possible.

Let's say you got full item build, or it doesn't really matter since the style of assassination is the same throughout the whole game unless you actually got your Trinity Force instead of boots build.

Without trinity force (mainly through the whole game)
As mentioned earlier, keep your ferocity bar charged up! What you want to do is pick an enemy off guard so look for mispositioned enemies, don't be afraid to ult to see if they're closeby. Run towards your enemy with Youmuu's Ghostblade and ultimate activated and right click on your enemy, but as soon as you right click also press your E ability towards the enemy which will result in you landing on a rooted enemy because your 5 ferocity E roots your enemies. Then proceed to using all your abilites and items (hydra) to proceed for the deadly shot, your 5 stack ferocity Q.

A little advanced
Keep in mind that the assassination style is the same but you can put a little twist to it. While jumping towards your enemy from your ultimate you're supposed to use your 5 stack ferocity E ability but also you can press twice in a row which will result that you land on your opponent with 1 ferocity stack already. If you can manage that then proceed to doing your 5 stack ferocity E right after the jump continued by an immidiate E then add W and Q to the combo which will result in you landing on a rooted enemy with 3 ferocity stacks available. On the meantime you will get 2 stacks real quick because of your ultimate passive since it automatically regenerates ferocity stacks so you will have 5 ferocity stacks right after using your 5 ferocity stacks up. This will allow you to use your 5 ferocity stacked Q immidiatly after landing and then use it again since it auto-generates. This will give a 100 % kill as long as the enemy isn't a full tank.

Summary: You'll land on a rooted enemy (E), then you will get 5 stack ferocity Q and then one more right after that.

With Trinity Force
Same concept but instead of using your E while landing use your Q. You can advance this with the same method as above. Use this method if you're certain the enemy can't flee away or if the enemy is really squishy then they will end up dying when you land on them.

Summary: Will result in a TRIPLE Q.