Place a 4-foot by 8-foot sheet of 3/4-inch-thick plywood on a pair of stable sawhorses.
Measure 12 inches in on each end of the plywood along one long 8-foot side, and make pencil marks. Cut a 12-inch-wide by 8-foot-long strip of plywood at these marks using a circular saw. Then, cut that 12-inch-wide, 8-foot-long strip in half, yielding two 4-foot-long sections.
Place your miter saw onto the remaining 3-foot by 8-foot section of plywood. The miter saw should be centered along the front, 8-foot edge of the plywood.
Measure the distance from the top of the plywood sheet up to the miter saw's table. Subtract 3/4 inch from this distance.
Set up your table saw's fence so that the fence is the distance you determined in the previous step away from the saw blade. Rip two 8-foot-long 2-inch by 6-inch boards to this width. Then, cut these two 8-foot boards into 4-foot-long sections using either your miter saw or circular saw.
Stand two of the ripped 2-by-6 boards on edge, parallel to one another about 8 inches apart, onto the plywood to the left of the miter saw. The rearward of these two boards should be parallel and in line with the miter saw's back fence. In this position, the two boards will cantilever off the plywood to the left of the miter saw. Connect these two boards to the plywood from underneath with a deck screw every 12 inches along the boards.
Repeat the previous step with the other two ripped 2-by-6 boards on the plywood to the right of the miter saw.
Mount one of the 12-inch by 4-foot strips of plywood onto the two parallel boards to the left of the miter saw. Connect the plywood strip to the boards using a deck screw every 12 inches along the joint. Repeat with the other strip on the right side of the miter saw.