Measure the distance from one side of the frame to the other with a tape measure. Write down the measurement.
Take another measurement approximately 1 foot down to verify no discrepancies exist between the two spots.
Purchase 1-inch by 4-inch boards from a lumberyard and cut them approximately ½ inch shorter than your width measurements. A queen bed requires five or six slats.
Sand the edges of the slats down to prevent splinters.
Place one slat at the top of the frame next to the headboard and space the additional slats approximately 20 inches apart on the frame.