Measure the width and length of your mattress. Purchase a sheet of 3/8-inch plywood according to those measurements or cut a sheet of plywood to fit. Most home improvement stores that sell plywood will do simple cuts for free. If not, mark the mattress measurements on the plywood sheet with a pen or pencil, then saw off excess materials using a handsaw or circular saw.
Slide the mattress off the bed and place it on the floor. Place the plywood sheet directly on top of the mattress foundation or box springs. Adjust the piece to fit so no edges show from underneath the mattress.
Place the mattress on top of the plywood sheet. Sit or lay on the mattress to test for firmness.