Lay two ropes parallel to one another on the ground from the beginning of the sidewalk to the end to mark the sides of the site. Use a tape measure to space them 4 feet apart plus 3 inches on each side for the frame.
Use a shovel to dig out growth and topsoil from between the two ropes, and pile it on a tarp nearby. Dig until the site has a uniform depth of 6 inches. Pitch the subsoil across the width of the site so one side is higher than the other. Dig down 1/4 inch for each foot of width at a smooth, consistent slope.
Press the flat end of a hand tamper over the subsoil to make it stable and free of air pockets.
Fill 4 inches of the site with 3/4 inch angular gravel and tamp it down. Continue to press the tamper over the base until the gravel doesn’t move under your feet. Stabilizing the base is an important step to prevent the cement from sinking over time.
Hammer 1-by-3 inch wood stakes in the ground every 3 feet around the inner perimeter of the site, so the top sits about 2 inches from ground level. Use the tape measure to verify that the higher side of the site sits 1 inch higher than the lower side.
Construct the frame from 2-by-4 inch boards, so the surface of the sidewalk sits 2 inches from ground level. Hold the first board on its side, so the top is even with the top of the wooden stakes. Secure the board to the stakes by hammering 2 nails through the back of the stake into the board.
Position the next board so the end is flush against the first board and the top is level with the stakes. Secure the board to the stakes by hammering nails through the outside of each stake into the board. Once the cement is poured and hardened, the frame boards will be easier to remove if the nails are driven from the outside in.
Continue securing boards to stakes until the entire site is framed and ready for the cement pour.