Measure and mark lumber using a carpenter’s square to ensure all lines are straight.
Cut lumber to size. Label pieces with letters on masking tape (to prevent damage to wood from pencil) and apply to each board as appropriate.
Lay both parts A side by side on the floor with the wide (3 1/2-inch actual size) sides facing up. Measure and make a pencil mark at the measuring tape positions 0, 2 1/2, 20, 40, 60, 77 1/2, and 80 inches on the inside of each board. (Use a carpenter’s square to draw straight lines across both boards at those positions.)
Use the lines as guides to drill two 7/64-inch pilot holes for #12 screws at each position except the 2 1/2 and 77 1/2-inch marks. (Make marks 1 inch from each side along each line.)
Return to the 2 1/2 and 77 1/2-inch positions on each part A and drill two holes the same way as before, but this time using a 9/64 drill bit. (Pilot holes for #16 screws.)
Take two of the parts B and lay together flat with wide sides up. Mark each board with a line 2 1/2 inches from each end in the same way you marked the drill points for parts A.
Use a 9/64 drill bit to drill two holes along each of those lines as with those drilled in A.
Turn parts A on their sides facing one another (pencil lines to the inside) approximately 60 inches apart.
Place the pre-drilled parts B (also on their narrow sides) at 0 and 80 on parts A, and the undrilled parts B at 20, 40, and 60 inches. Fasten them to parts A using #12 screws. This is the base bed frame.
Put one part C (legs) into each of the four inside corners of the assembled base frame. Fasten the frame to the legs on each of the two inside faces through the remaining pilot holes using #16 wood screws.
Lay both parts D flat (wide sides facing up) and mark a line on each at 4 inches from each end and again at the center point of the board (30 3/4 inches). Mark two spots for drill holes on that line at 2 3/4 inch from one edge and again at 4 1/4 inches down (from the same edge).
Lay both parts E flat (wide sides facing up) and mark a line on each at 4 inches from each end and again at the center point of the board (40 inches). Mark spots for drill holes on lines exactly as for parts D.
Use a 5/64 drill bit with an appropriately sized countersink to drill countersunk pilot holes in each of the marked spots on all parts D and E.
Start at the same edge as used when measuring for drill holes and mark a line parallel to the edge at 1 3/4 from the edge on all parts D and E. (These are your guidelines for attaching the outer frame to the inner, base frame.)
Turn the bed frame over so that it rests upright on the legs. Attach parts D and E to the base frame with #8 wood screws through the countersunk pilot holes.
To finish, fill countersunk holes with dowel plugs of the same size. Sand all parts smooth (with the grain), then stain or paint as desired.