Measure the length and the width of your mattress using measuring tape.
Cut a piece of plywood that matches the length and width you measured, using a circular saw. Some home improvement stores will cut plywood for you if you purchase it there. If you cut the plywood yourself, properly support the bulk of the plywood sheet as you push it against the saw. This prevents the saw from binding. Cut carefully, and if you are not sure of your ability to cut the sheets of plywood, find a service that is willing to make the cuts for you.
Place five cinder blocks on the floor, the openings facing up. They should be arranged so that there will be one block at each corner of the bed and one in the center. If you are building a base for a larger bed, use two or more blocks along the center line of the bed. The cinder blocks hold the platform about 8 inches off of the ground.
Place the piece of plywood on top of the blocks.
Place your box spring on top of the plywood and your mattress on top of the box spring.