Cut the wood for the box frame. Use a tape measure and pencil to mark two 10 by 51-inch sections and two 10 by 25-inch sections on the 4 by 8 foot piece of plywood. Use a circular saw to cut each piece at the pencil mark.
Attach the box frame. Create a rectangular box frame that measures 51 by 25 inches. Have someone hold the pieces together as you drill three screws into each side to secure the frame together.
Mark the mattress rails. On the outside of the long sides of the frame, measure 2 ½ inches down from the top and mark with a pencil in five locations along the length of the frame. On the inside of the long sides of the frame, measure 2 inches down from the top and mark with a pencil in several locations. Use a level to draw a straight, even line between each mark.
Attach the mattress rails. Have a helper hold each 2 by 2-inch piece of wood along the inside of the bed frame so the top hits just under the pencil marks. Drill screws at the 2 ½-inch marks along the length of the outside of the bed frame into the mattress rails.
Attach the plywood bottom. Lay the 24 by 48-inch sheet of plywood on the mattress rails and attach with 2 screws on each side.
Measure and cut the batting. Roll out the batting and fold over so it is twice as thick. Measure and cut the batting in 1 ½ lengths of batting that are each 15 inches wide.
Attach the batting. With the folded edge of batting placed over the top of the bed frame, attach it to the inside of the frame with a staple gun. Begin at one end of the frame and work around the entire perimeter, trimming the batting at the corners, if needed, for a more secure fit. Add the second section of batting after the first runs out before flipping the bed over to attach the bottom of the batting to the frame. Pull the batting tightly around the bottom edge when securing to ensure a proper fit.
Measure and cut the fabric. Roll out the fabric and cut it to measure approximately 4 ½ yards long by 16 inches wide.
Attach fabric. Begin in the center of one of the ends of the bed. Staple the fabric over the batting at the bottom of the frame, working around the bed and folding the corners in as they are encountered. Turn the bed over and begin stapling the fabric over the top of the frame and around the entire perimeter.
Insert the mattress. Lay a crib mattress on the mattress rails in the finished bed frame.