FIRST- take an old bra that already fits you and cut 1 inch slits in the front to fit the cups for the pump.  That is all you need!
This type of thing is a must have BUT I found this very very uncomfortable.  The fitting is done by both 2 zippers and velcro.  I am medium sized and you cannot adjust how far spaced the cups are, so it makes my cups go out to the sides, it looks crazy and feels crazy.  But worse is the ruff material around the breasts which irritate my breast the entire time.  The whole thing slides down at times.  I use it when my homemade comfortable, well fitting, cheap one is in the wash and every time I put it on I remember how much I dislike it.  It get great reviews because being hands free is a must.