Start by marking the dimensions of the pad on the ground and remove 9 in. of soil from within the marks. Fill the area with 6 in. of compacted gravel, tamping after each 2-in. layer. Top with a 3-in. layer of sand.
Build a frame of 2x6 lumber, ensuring that it is square, and place it over the area. Drive stakes around the outside perimeter to keep the frame from bowing outward with the weight of the concrete. Check the tops for level in all directions. To maintain the frame’s position, tack it to the stakes. Place bricks or 3-in.-dia. stones within the area to support three strips of rebar and mesh (leaving about 2 in. of space between the frame and the rebar or mesh).
Mix concrete (we used 12 80-pound bags of concrete mix), following the directions on the bag. Pour to fill the boxed area, being careful not to disturb the structural metal. Use a flat trowel to smooth the surface, and screed the top using a straight-edged board spanning the frame. To ensure proper curing, prevent the concrete from drying too quickly by periodically misting water on the surface and keeping it covered with plastic. The setting time will depend on the concrete mixture, humidity and temperatures.