Remove the mattress from the bed frame. Measure the width inside the frame where the mattress rests. Subtract 1/2 inch from this measurement to find the length of slats you need. If the frame width is 39 inches, you will cut 38 1/2-inch slats.
Measure the slat length and mark it on the 1 by 3 boards. Cut the slats with a hand saw, measuring and marking each new length, until you have 12 slats for each bunk bed.
Smooth off the ends of each slat, first with a wood file, then with medium sandpaper to remove any slivers or rough spots.
Place the slats cross-ways on the bed frame. Space the slats evenly from the top of the frame to the bottom. Replace the bed mattress.