Run vertical strips of masking tape down the full height of the walls at 2-foot intervals. Dog-ear the corners so you can peel off the tape. You'll be able to remove the foam rubber much more easily than if it were glued to the wall.
Swab rubber cement onto the tape. Do only a few rows at a time. Otherwise, the cement on the last strips of tape will dry before you can get the foam rubber on them.
Hold the foam rubber pads up horizontally and stick them to the glued surfaces of the tape. It is better for the pads to run in separate directions from the tape so you do not have to worry about the tape lining up with the foam rubber at the proper intervals. Do this all around the walls of the room. You can soundproof the ceiling in the same manner.
Place several layers of old carpet, mats or both on the floor, especially under the drum set and amps. This isn't as important if you have your band room in a basement. But if you have family or neighbors downstairs, this step is absolutely vital. Foam rubber is not durable enough to last on the floor, but put down as many mats or carpet layers as you want.
Place a pillow inside the bass drum. This is important because it is probably the loudest instrument you have.