Outline the borders of the shop floor on the site with landscaping paint. Excavate the shop site with shovels, removing 12 inches of soil from the ground. Dig until the floor of the recess is level.
Use a vibrating compactor machine to force out air and excess moisture from the leveled ground. Go over the site three times or until the ground resists the pressure.
Install a wood frame round the inner walls of the site. Insert wood stakes against the soil walls about every 3 feet and in corners. You'll need to stack three boards on their sides to cover the depth of the foundation and slab. The top of the frame will form the surface of your floor. Place a level on each side and adjust the frame until it is even before nailing the boards. Use a hammer and nails to secure 2-by-4 boards to the stakes.
Set a vapor barrier over the bottom of the recess to block moisture from seeping through the ground.
Add two 4-inch layers of aggregate over the barrier. Compact the aggregate between each layer so the rocks are packed snug to form a strong base.
Refer to local building codes to see where you are required to place rebar. Set the rebar as instructed to reinforce the concrete.
Place ¼-inch foam, rubber or cork expansion strips in a grid over the surface. Space them about 2 feet apart for every inch of the slab's depth. If you are pouring a 4-inch slab, grid the expansion joints every 8 square feet.
Use a concrete mixer to prepare a batch of concrete so it is thick yet pourable. Pour the concrete over the framed site, spreading it around the first section of the grid with the back of a square shovel. Stand the expansion joint up vertically and pour the adjoining side to hold it in place, keeping the top of the joint clear of concrete. Continue to pour each section this way, pressing the expansion joints against the concrete section after each pour.
Screed the slab by dragging a screed board over the frame to remove excess and fill any low areas. Push the board back and forth until the surface is level. Push a bull float along the surface to make it as smooth as you possibly can. Stop working the surface once water starts to appear on top.
Drape wet burlap over the slab and give it seven days to dry. Moisten the burlap throughout the time to slow the drying process.
Lay expansion joint sealant over the top of the joints to prevent moisture from running down through the sides of the joints.