Measure the dimensions of your closet. Find the height and width of the ceiling, the three walls, and the inside of the door panel.
Measure, cut, and combine the fiberglass pieces into five panels. One for the ceiling, one for each of the three walls, and one for the inside of the door panel. The panels should be cut two inches shy of the dimensions of your closet, but should effectively cover the entire surface.
Completely wrap each of the five fiberglass panels in felt. Staple the felt to the fiberglass to secure the felt in place. You should have five separate panels of fiberglass completely covered in felt.
Staple the fiberglass panels to the three walls, ceiling, and door.