Cut four pieces of plywood board into 80-inch-by-40-inch rectangles.
Lay the boards on the floor, in pairs. Line the boards up so the corners are level.
Place a hinge in an overlapping position over the first two boards, approximately 10 inches from the top. Put a second hinge in an overlapping position approximately 10 inches from the bottom. Push a sharp pencil through the screw-mounting holes on the hinge to leave a mark on the boards. Repeat this process with the other two boards so each pair of boards have mounting holes plotted on them.
Drill a 1/8-inch wide hole over each pencil mark to make mounting holes for the hinges.
Screw in the hinges using a Phillips-head screwdriver and 4-inch screws.
Lift up the first pair of boards. Support each board as you lift so you don't put undue strain on the hinges.
Lower the boards onto their backs so the hinged sides are facing down. Repeat this process for both pairs of boards.
Cut the foam squares into wedges. Slice from the top-corner to the bottom right corner. Getting a wedge shape is essential. Peaked surfaces are essential in reducing sound-reflection as they absorb the sound waves, and prevent them from moving around. Foam has absorbent qualities that heighten the effect of the wedges.
Spray each foam wedge with fire-retardant spray, and then leave them to dry for one hour. It's crucial that you apply the fire retardant after cutting the foam. If you treat the outside, then cut the foam open, the untreated foam will be exposed.
Glue the foam wedges onto the inside of plywood boards. It's not necessary to cover the entire surface of the board, but you need to cover most of it. You know have two pairs of folding walls. To insulate the sound, position the boards around the source of the sound, for example the amp or the drum kit. When you want to put the boards away, fold them in. The foam will compress as you do this.