You start wiht volley farming passively as you buy your items.
If the enemy has alot of harass try to got 2 dorans for early game damage.
Use hawkshot to watch for ganks.

Around 20:00 minutes in you should have your bloodthirster and zeal with atleast 60 or more creeps.if you are dying alot substitute your second phantom dancer for an aegis of the leigon or warmogs.Teamfighting takes place around midgame so do not waste ur ulti trying to get those across the map snipes.

By now you should have your infinity edge and phantom dancer and atleast 100 or more creeps.
The teamfights get serious now and if ur support and tank arent doing so well you are going to have to let tham farm a bit more try to hold off teamfights if team is not ready and Hawkshot to check for 4 or 5 man ganks.