Step One: Cuts

To make these directions easier, I'll just relate the construction of a single box; repeat four times for a whole bed. The dimensions here are for a queen mattress, which is 60" by 80." Refer to this Wikipedia chart and adapt for the dimensions of your own mattress. A single (twin), could be supported with just two boxes instead of four. Cut one panel of 3/4" plywood at 32" x 42" (the top); one 3/4" panel at 32" x 14" (side one); one 1/2" panel at 32" x 14" (side two); one 1/2" panel at 40-3/4" x 14" (the stiffening rib); one 3/4" panel at 40-3/4" x 3" (the door track); and one 1/2" panel at 40-1/2" x 18" (the shelf).

To cut square pieces out of sheet goods with just a circular saw, clamp down a straight edge, forming a temporary fence. Measure your circular saw from the edge of the baseplate to the face of the blade, and offset the straight edge from your desired cut line by that same amount. When cutting, push the straight side of the base plate of the saw against the straight edge, guiding your cut. For clean cuts in sheet goods, use a 60-tooth finishing blade with carbide teeth, which will cut smoothly with minimum chip-out. For cross cuts, clamp down a speed square, registered against the long edge of the work piece, as your saw guide. All the measuring and clamping and re-arranging is kind of tedious, but important for accuracy. Take the time to make precise cuts, and later joinery will be painless.