Measure the length, width and height of the frame interior. Choose a mattress as close to those dimensions as possible. A pillow top or other thick mattress is preferable; if the mattress is too thin, the frame will rise above it on all sides. A box spring is not necessary.
Place the mattress inside the frame. Center it in the middle, leaving equal distance on each side if the mattress does not take up the entire space.
Place a mattress topper over the mattress if it isn't thick enough to reach the top of the frame. Alternatively, cut one or more sheets of plywood to the inside dimensions of the frame with a circular saw. Place the plywood sheets beneath the mattress until the top of the mattress is level with or slightly taller than the top of the frame.
Cover the bed with a fitted sheet, flat sheet, blanket and comforter or quilt. Tuck the edges of the flat sheet, blanket and comforter or quilt between the mattress and frame.