Spread the shower curtain or shower curtain liner out on the floor. If the curtain is used, make sure it is dry and free of mildew, soap scum and other stains, as these things can damage the mattress.
Remove the fitted sheet from the mattress and stand it on its side. Flip the mattress onto the shower curtain, top-side down. Center it on the material, leaving as much excess plastic as possible on all sides.
Fold the sides of the shower curtain up over the mattress, securing it with duct tape. Pull the plastic taut so that there are no wrinkles in the material.
Place the mattress back on the box spring. Put the fitted sheet back over the mattress and remake the bed.