Position two of the 43-inch boards so they are parallel, resting on their 1½-inch edges and 15½ inches apart. The ends of both boards should be aligned.
Set two 15½-inch boards between the 43-inch boards perpendicularly, so their 5½-inch surfaces are flush with the ends of the 43-inch boards. You should have a rectangle with an outside perimeter of 18½ inches by 43 inches.
User your screw gun to screw through the 43-inch boards into the ends of the 15½-inch boards. Use eight screws for this task; they should enter the center of the 15½-inch boards, 1 inch away from each 1½-inch side. This is the frame for the top of your miter saw stand. All of the corners must be flush.
Position a 33-inch board perpendicularly inside each corner of the frame so the ends of these boards are flush with one side of the frame. Screw four screws through each corner of the frame and into the 33-inch boards. They should enter the center of the 33-inch boards, and be spaced 3 inches apart. You have just fastened the legs to your stand.
Position your stand upright so the frame is on top. Screw the last two 43-inch boards to the legs. They should be perpendicular to the legs and 16 inches above the ground. The ends of the 43-inch boards should be flush with the outside edges of the legs. Use eight screws for this task.
Lay your plywood on top of the frame so all of the edges are flush. Screw through the plywood and into the frame. Use eight screws for this task.
Position your miter saw on the center of the plywood. Use your tape measure to center it accurately. Bolt the miter saw to the stand using the bolts that came with it.