Remove the box spring from the bed frame.
Measure the width of the frame between the left and right bed rails to determine how long the support slats must be.
Cut the bed slats from ¾-by-3-inch lumber. You'll want enough slats to space them around 4 inches apart within the bed frame.
Lay the wooden slats across the left rail, center support beam and right rail. If the frame allows it, you can screw the slats in place to improve stability.
Replace the box spring in the bed frame.