Nail together four 4-foot lengths of 2x12 Douglas fir boards to make a square. Set on the graded area. Build a second square out of 2x6s cut at 4-foot lengths (Image 1). Set one square on top of the other, and secure them with 2-1/2'-long forming stakes set on the outside of the form. Use two stakes per side (Image 2). These will also secure the form to the ground, keeping it from shifting. Make sure the square sits level.
Repeat same steps for the inside frame, but make it 8 to 10 inches smaller (Image 1). Set inside the exterior frame, making sure it sits even on each corner, creating a 4- to 5-inch gap (Image 2). Stake in place, installing the stakes on the inside of the frame. Make sure the entire frame is level.
Drive 20-inch pieces of rebar into the corners of the square as well as along every foot inside the frame (Image 1). Make sure the rebar is securely in the ground and sits below the edge of the frame.
Run 4-foot pieces of rebar horizontally between the vertical rebar. Tie together with steel metal wire. This will make a grid (Image 2). Cut wire with wire cutters.
For extra reinforcement, add wire mesh or matting. Cut pieces to fit inside the form. Tie to the rebar grid with wire.
Pour concrete inside the frame (Image 1). Tamp down the concrete so all holes are filled; you don't want any spaces inside the form. Smooth off the top with a trowel. Tap forms while concrete is still wet — this releases the concrete from the forms while drying. Let dry overnight (Image 2).
Once the concrete is dry and set up, carefully strip the forms.
Add a thin layer of mortar along the outside of the frame to add a smooth finish (Image 1). If desired, cut pieces of slate to fit the top of the frame and mortar in place (Image 2).
Add gravel about three quarters of the way full, so the top of the fire bowl will be flush with the top of the concrete frame.
Add or subtract gravel accordingly to bring the bowl to the correct height.
Add a large cobble, such as Mexican pebble, along the outsides of the bowl until only the top rim of the bowl is exposed.
These homeowners had an old metal fire bowl that was headed for the trash. Instead of throwing out the entire thing, they decided to reuse the metal bowl. By creating a square concrete frame and filling it with rock, they set the bowl in place and surround with decorative rock, hiding the outside of the bowl completely. Add a little flagstone or slate to cap the concrete frame, and this fire feature is complete.