Cut each of the treated 2-inch by 12-inch boards to a length of 72 inches, using a circular saw. Stand two of the boards on their side so they are parallel to each other and 7 feet apart.
Place the remaining three boards on their side perpendicular to the first two. Move these boards so the entire layout is a square with the fifth board running through the center.
Fasten the frame together by driving three 16d nails through the outer boards and into the end of each of the three boards in between. Fasten each junction and leave each nail head 1/8-inch away from the surface of the board for easy removal later.
Lay the two pieces of plywood on your frame. Arrange the plywood so the seam between them runs along the length of the center board in the frame. Fasten the plywood to the frame, using roofing nails.
Apply a grid of construction adhesive to the top of the plywood deck with a caulking gun. Space out the beads of adhesive so they run every 3 inches along the plywood in each direction.
Lay the outdoor carpet over the grid of adhesive on the plywood decking. Pull the carpet tight and press it down onto the deck. Run a wallpaper roller repeatedly in each direction to further compress the carpet into the adhesive. Let the adhesive dry overnight.