I need help with replacing Vacuum booster on 97 cavalier.

[daveb2012]

I haven't ever done this but im going to figure it out because I dont have enough money to take my car to a shop. its a 97 cavalier.
The main problem I have had with it almost 1 year now, is that it does this thing where if im going less then say 8Mph the brakes seems to slam on when i hit them, I have to also hit the brake pedal really hard. pads and rotars are new so I can single that out I wasted what $ i had on taking the mechanics advice to change them to fix this problem it didnt help at all.

I was wondering if you guys here could point me in the right diretion on replacing this part and if sounds like this part is whats broke. I cannot find any website that explains how to do it, not sure how hard it is, or what it even looks like.

Also the car is the worst when it hasn't been drove in a few days (at first that is) when it warms up its not as bad, I just hate how when im going slow at I brake at first they slam on, I thought maybe their was something that causes slack on my wheels, because if I mark on down ward hill once I let my foot of the brake the car rolls about 1 - 1.5" Im not sure if this is normal, the tires dont physically roll. just the wheel slack. does this still sound like vacuum pump, thanks for any help guys.