Remove the mattress and box springs from the bed frame. Move the frame to the center of the room.
Lay out the slats under the bed in a horizontal position. Spread the slats out so they are approximately the same distance apart.
Remove one slat from beneath the bed and lay it across the frame near the bottom of the bed. Lay the slat horizontally without placing it at an angle. Push down on the slat until it rests on the frame.
Tap the slat with the mallet to even out the ends if the slat is at a slight angle. Tap the top of the slat down to seat it correctly if your slat has a tight fit.
Add the rest of the slats in order, keeping them the same distance apart, until you place the last slat at the top of the bed frame.
Place the bed spring on top of the slats and the mattress on the box spring.