To build a concrete slab form, drive stakes into the ground near each corner of the slab. Use a circular saw to cut framing lumber so that each sideboard is 3 inches longer than the length of the finished slab. Make sure the end board matches the exact width of the finished slab.
Taking into account the correct height of the slab, nail a sideboard to one of the corner stakes. Trim the high side of the board to match the height of the plate. To adjust the height of the form, tap the stake a little deeper using a sledgehammer. Use a carpenter’s level to set the form board to the appropriate level, and then nail it to the other corner stake. Stretch a masonry string across the top of the leveled form board.
Cut lengths of lumbers long enough to go from the top of the form board to about 8 inches below the ground. Make sure that the lumber is enough to brace the form board every 2 feet along its length. Drive the braces into the ground along and against the outside part of the form board. Ensure the board is level, and then nail the braces in place, with nails driving through the brace into the form board.
Attach an edge board to the brace form board, and level it using a carpenter’s level. Make sure that the two boards meet at a 90-degree angle. Brace the second board, and follow the same steps as in the first board. Add gravel to fill the inside of the form, and then attach the final sideboard.