Use an appropriately sized board to build the waterbed headboard. Waterbed headboards are typically constructing using a 1-by-3 board.
Measure out the headboard length on the board. Mark the length carefully.
Cut through the board using a power saw at the place you marked. Sand the edges to smooth them over.
Mark the midpoint of the headboard from the back. Then, mark the midpoint of either side. This is where the headboard will be attached to the bed frame.
Place the headboard on top of the back of the bed frame so that the two pieces come together lengthwise.
Ask someone to hold the headboard while you attach the two headboard supports, or brackets, from behind on the spots you marked. Use a power screwdriver and wood screws.