Spray paint the outline of the slab onto the ground.
Excavate the area inside the spray paint. Use hand tools like picks and shovels for smaller slabs; for a large slab, you can rent a 1 1/2-ton excavator. Dig a hole that goes beneath the frost line to prevent the ground from heaving up during freeze/thaw cycles and damaging the slab. Your local zoning board can tell you the depth of your frost line.
Drive wooden stakes into the ground every three feet around the perimeter of the slab.
Nail 2-foot by 4-foot boards to the stakes. Measure and cut the boards so the ends meet in the middle of a stake and not in the gaps between the stakes.
Cover the dirt at the bottom of the hole with a vapor barrier. This is a plastic sheet that prevents moisture from seeping up from the ground.
Cover the vapor barrier with sand or gravel, following local building codes regarding which material to use and the depth of the layer.
Place wire mesh or rebar on the sand or gravel base, again following local building codes.
Mix a batch of concrete, following the directions on the packaging. For smaller slabs, mix the concrete in a wheelbarrow; for larger slabs, rent a cement mixer.
Fill the hole with concrete until the concrete goes over the top edge of the frame.
Screed the slab. This smooths out the concrete and exposes low spots in the pour. With the help of a partner, drag a 2-foot by 4-foot board across the top of the frame with a side-to-side motion. Fill in any low spots in the pour, and screed the slab again.
Finish the surface of the concrete with a bull float. This is a long-handled device with a flat surface used to finish concrete. Hold the bull float on the near edge of the concrete, tilted with the back edge of the float in the air. Push the float across the concrete. Lay the float flat on the concrete when you reach the other end, and pull it back toward you.
Cover the concrete with plastic sheathing. Remove the sheathing and add enough water to keep the concrete damp once a day, then cover the slab with the sheathing again. Continue doing this for a week, then allow the concrete to cure for four to five days.
Lift the plastic sheet off the concrete and remove the wooden frame.