Measure the dimensions of your closet's walls individually, notating height and width. Multiply the height and width of each wall to get the space's area. Repeat the process for the door, ceiling and floor. Add the area of all four walls, the door and ceiling to determine the amount of materials you will need.
Cut your plywood into sections to exactly match the dimensions of your walls, door, ceiling and floor. You may need to use a jigsaw for more difficult center cuts, such as to allow access to an air vent or electrical outlet. Remove an extra inch from the top and bottom edges of all four wall pieces to allow room for the ceiling and floor sections.
Attach your plywood panels to the walls. Secure each with at least eight screws around the perimeter. Carefully attach the ceiling and floor pieces, and secure with screws.
Trim your soundproofing foam to size using a box cutter. Create separate pieces for the walls and ceiling, this material is not meant to be installed as flooring. Remember to cut out holes for air vents or electrical outlets.
Attach your soundproofing foam to the walls and ceiling by removing the film on the back and carefully pressing it into place. With each section of foam, begin pressing onto the wall at the top and work down to avoid creating air bubbles.