Measure the box springs or platform. Determine the mattress size before purchasing elements of the bed. Standard mattress sizes are California king at 72-by-84 inches; king at 76-by-80 inches; queen at 60-by-80 inches; full or double at 54-by-75 inches; and twin or single at 39-by-75 inches.
Lay the high density foam on a flat surface. Mark the measurements directly onto the foam using a yardstick and permanent marker.
Cut the foam with the carving knife. Use a yardstick or long straight edge to cut evenly. Do not try to cut all the way through the foam with one stroke. Instead, make several long strokes until the knife breaks through to the bottom of the foam.
Place the egg-crate topper on top of the high-density foam. Align the edges carefully and smooth it out so the egg-crate topper lies flat. If you prefer a smooth top, simply turn it over with the flat side up.
Slide the layered foam pads into the zippered mattress cover. Zip it closed and place your foam mattress on top of the box springs or platform bed. Add a memory foam topper if desired.