Measure the width and length of your mattress. Add 2 inches to each of those dimensions. Use these adjusted measurements as the guidelines for cutting your 2-by-6 boards. Cut two boards equal to your adjusted width and two boards that equal your adjusted length.
Place the boards together on the floor to from a rectangular mattress frame. Place a thin amount of wood glue on the end of one of the boards of the correct width, and join it to one of the longer boards to create one corner of the bed frame. Repeat on the remaining corners. Turn the frame so the long sides are on the left and right.
Load your nail gun with 2-inch nails. Further secure your corners by firing three nails down the length of each corner.
Use a measuring tape to measure the length of the frame by measuring from the inside edge of the top board to the inside edge of the bottom board. Cut your remaining 2-by-6 board to this length.
Apply spray adhesive to the outer face of the frame boards. Wrap the bed frame with batting. Use the same method you would use to wrap a gift. Secure the batting in place by stapling around the inside perimeter of each board.
Fit the board you cut in Step 4 into the center of the bed frame to create a center support beam. Secure the board in place by firing nails through the top frame board and batting into the cross support beam. Repeat with the bottom board.
Wrap the frame boards with upholstery fabric, using the same method used to apply the batting. Pull the fabric taut before stapling to the inside of the bed frame. This will help keep the finish smooth.
Measure the length from the center support board to the outside, right rail of the bed frame. Cut eight pieces of plywood equal to this length by 8 inches wide. Repeat on the left side of the center support beam.
Space the plywood boards evenly along the right side of the bed frame. Place them on top of the frame box built in the previous steps. Secure the boards in place by firing nails through the top of the board into the center rail and into the outside, right rail. Repeat on the left side.
Cut your second plywood board into a rectangle that measures 4 inches longer than the bed frame on all sides. Cover the front of the plywood sheet with spray adhesive. Wrap the plywood sheet with batting and secure the batting to the back by stapling around the perimeter.
Unroll the remaining upholstery fabric facedown on the work surface. Place the plywood sheet staple-side-up on the fabric. Trace out a fabric rectangle about 6 inches longer than the plywood sheet on all sides. Wrap the fabric around the board, pulling it taut before stapling it into place.
Place the upholstered sheet on top of the frame. Nail through the fabric into the bed frame rails to secure the plywood sheet in place, if necessary.