To start off, with AP rengar I will usually go top or bot. This is because if I go mid there are no real good brushes to jump out of so you will have no surprise factor for the enemy. If you go top or bot in urf you will always have a teammate due to the meta so let your teammate poke them while you farm or wait in a bush.

Your W is your main source of damage on AP rengar. You want to max your W first then max your Q due to the fact that your Q works with lich bane so you could get even more burst on them!

In laning phase, I usually don't go in for kills unless I have built Luden's Echo. The reason fot this is, Luden's Echo gives so much burst and it works with your W so you could assassinate people even faster.

Now, if you want to get a kill make sure to build up 5 ferocity before going in!!!!
This is really important because it makes you do twice the damage you would normally do with your W due to the fact that you could cast it twice.

So pretty much all I do in laning phase is farm. If they have ranged champions I sit in a bush and wait for them to come in range of my leap then I jump on them and use my W for some nice damage.

A nice way to farm is to sit in a bush and jump on a minion that is low, then run back into the bush. Another way is to jump from the bush then use w to clear the mage minions.