Step 6. Assemble the Base
Place the 23" base sides between the 22-1/2" base ends, standing on their narrow edges, to form a base measuring 22-1/2 x 26" (the same size as the plywood floor).
Drill two pilot holes in the ends of each 22-1/2" piece, evenly spaced, with a 3/16" drill bit with a countersink. Screw the frame together with the drill/driver using 3" exterior wood screws.
Depending on what type of screw you're using, pilot holes should be just slightly smaller than the screw.
The idea is to allow the hole to be big enough for the screw to pass through without splitting the wood, but at the same time, small enough so that the threads grip firmly.