Measure and lay out markers where the four corners of your shed will be constructed. Dig out all of the dirt within the markers 6 inches deep.
Create a concrete slab form by placing 1-inch-by 6-inch-by-8-foot boards around the perimeter of the shed -- one on each side.
Hammer a 1-foot wood stake behind the form boards every 12 inches. Attach the stakes to the form boards using double-headed nails.
Fill the inside of the slab area with 2 inches of sand. Wet and rake the sand smooth and flat. The moisture in the sand will prevent the sand from drawing the moisture out of the concrete.
Pour in a ready-mix concrete over the sand, 4 inches deep. Using a long-handled trowel, smooth out the concrete slab until it is nice and even.
Allow your concrete slab to cure completely -- usually a day. Remove the stakes and the form boards and build your shed.