Lay the two 7-inch boards on their 1-by-7-inch edges, parallel and 10 inches apart. Place the 12-inch board perpendicularly against the ends of these boards so it rests on its 1-by-12-inch edge. The two corners should be flush. Screw the 12-inch board to the 7-inch boards, using four screws. Two screws should enter the each 7-inch board's end, with each pair of screws spaced 3 inches apart. The screws are each 1-inch away from the edges of the board.
Lay your plywood on top of the three boards so all of the edges are flush. Screw the plywood to the boards using two screws for each 8-inch edge, and three screws for the 12-inch edge.
Turn the plywood and three boards over so the three boards are on top of the plywood. Lay the last sheet of plywood on top of the boards so all of the edges are flush and screw it down following the same process in the last step.