The best way is to put a clear line on the nipple and put the other end in a bottle/cup of fluid that way when it sucks back in it will suck up fluid and not air. When there is no more bubbles in the line your done. I did it all by my lonesome this way. This pic should help you get the idea. Just make sure to keep the line submerged and keep the resivoir full or you risk sucking more air into the system.
(note I didn't have a clear line so I used a black hose, a clear line will allow you to see the air bubbles)