Use a table saw and trim two of the 2-by-6 boards so they are 6 inches longer than your mattress set and the other two so they are 6 inches wider than your mattress set. Run a tack cloth all over these boards to remove any sawdust.
Turn one of the shorter boards and one of the longer boards up on their long narrow edges. Butt the short narrow edge of the shorter piece up against the flat of the longer board at the end so that the top, bottom and outer edges are all aligned. The boards should form a 90-degree angle.
Glue the two boards together with wood glue and clamp them in place. Hammer a straight line of nails down the glued seam and release the clamp. Repeat Steps 2 and 3 for the remaining short and long boards.
Bring the two sets of boards together to form a rectangle. Line up the boards so the edges of the shorter board are on the inside and the edges of the longer boards are on the outside. Glue, clamp and nail the remaining corners and you did the first two.
Fill in the nail holes with wood putty and wait two to three hours. Sand the whole frame smooth and then rub it down with a fresh tack cloth. Stain or paint the frame as you wish and let it dry, then stack the box spring and mattress inside.
Install a set of wooden slats evenly across the width of the frame from top to bottom for additional support or to raise the mattresses. These should slide in easily with the narrow edges facing upward and be nailed in place. The length and number of slats will vary based upon your bed size.