Turn three planks of 75-inch 2-by-12-inch lumber so that they are lying lengthwise on a 2-inch edge. Space them so they are 20 inches apart and are parallel with each other. These are the base planks of the single-sized platform slat bed.
Lay 21 planks of 50-inch 1-by-3 lumber across the three base planks, with one-half-inch space between them. These are the bed slats, and they should be positioned so that there is 5 inches overlap on the ends of the each slat from the outer two base planks.
Clamp the first slat in position on top of the three base planks. Drill a pilot hole, sized for 1 1/2-inch wood screws, through the slat into each of the base planks. Secure the slat in place with a 1 1/2-inch screw through each of the pilot holes.
Repeat for each of the slats, ensuring before you clamp it down that the slat is perfectly aligned with the one before it. This is a single-sized platform slat bed, where 5 inches of the slats on either side of the mattress will be visible once a mattress is placed on the bed.