Measure the dimensions of the item you want to place in the hole.
Draw the outline of the hole on the mattress to minimize mistakes when cutting. Carefully consider the placement of the hole. If you intend to use the hole to store an item, place the hole in a hidden place that does not affect the aesthetics of the mattress or your comfort when using it. If it is for a pregnant woman, try having her lie down on the mattress to determine where the hole should be placed.
Use a sharp, non-serrated knife or a saw to cut the hole. If you have a foam mattress, use an electric knife for smoother cutting.
Remove any stuffing in the hole and sew fabric on the exposed area of the mattress to prevent any spring or stuffing from poking out, if necessary.