Cut plywood into panels as labeled in the diagram below. Making sequential cuts will make this process easier with one person. Start with a fresh blade and use a rip fence or cutting guide*. The panels can also be cut on a table saw if help is available to hold the sheet as it is pushed across the table. The dimensions shown are the finished lengths, so make sure you place the blade on the correct side of the cut mark prior to ripping. The cross hatched areas are drops of wood that can be discarded. Reserve all other pieces.
In a pinch, a straight 2x4 and two C-clamps can make an improvised cutting guide.
Once the panels are cut, start assembling the four pieces. Wood glue and screws will be used to secure the panels together at the corners. Drill three pilot holes in each end of the side panels and start the screws on the flat surface, then assemble the four pieces on the floor (12 screws total). Make sure that each piece is flush to the next. Once these four pieces are joined, the four gussets will be installed flush to the bottom of each frame corner. Again use glue and three wood screws on each side (24 screws total) making sure that the screw heads are flush or slightly recessed*.
Before installing the gussets, make sure the frame is square by checking the diagonal measurements. If the frame is square, both measurements will be equal.
Cut the 4x4 into four 11-1/2"-long pieces. The back of the 4x4 will be notched out so that it wraps the frame corner, is flush at the bottom and protrudes 1 1/2" above the top of the frame. Start by marking the cut-out with a 10" line 2" in on two adjacent sides. Set your saw blade to a 2" depth and make these two 10"-long cuts (cuts 1 and 2).
Next, make the two perpendicular cuts with the same saw depth (10" from the end, cuts 3 and 4). There will now only be a small piece of wood connecting the cut-out corner and the drop. Using a vibratory saw or a chisel, remove the final wood and clean up the inside corner. Lastly, cut the back of the 1 1/2" top at a 45-degree angle. Glue the interior faces of the corner and finish nail to the bed frame.
Install the two plywood ledger strips even with the bottom of the bed frame sides. Use wood glue and install 1-1/4" screws 6" on center through the face of the ledger into the side panel. Measure the distance between the bed frame sides (in this case, 6' 5") and cut seven 2x4s to this length. Next, install the cross slats 12" on center with one screw into the ledger on each end. (We used 1x4s for our bed and added a middle leg to support the weight. But if you want to skip using a middle leg, make the cross slats using 2x4s.)
If solid legs have threaded rod in the tops, install the leg-mounting thread into the corner gusset on each side. If the mounting thread is accessible, install a washer and nut, and tighten. If the mounting thread is not accessible, use a quality construction adhesive on the thread, hole and top*.
Undersize the hole for the threaded rod by 1/16" for a tight fit. The rod can be threaded into the wood by spinning the leg clockwise.
Cut (8) pieces of trim to fit between the corner posts and install top and bottom around the perimeter of the frame with 15-gauge finish nails. Caulk all joints and nail holes with a painter's caulk and allow to dry for 30 minutes. While the caulk is drying, cut two pieces of 1/4” sheathing to go over the 2x4 cross bars. If you are using a box spring, you can skip this step but your mattress will likely sit above the frame instead of within it. Finish up by painting the frame with your color of choice."