Design the dimensions and shape of the slab you desire on paper.
Build the skeleton (perimeter) of the mold using the 2x4's (if you require a slab deeper than 4 inches, use larger planks). Apply stakes outside the mold to prevent a blow out from just-poured concrete. Concrete is incredibly heavy and applies a lot of pressure on the perimeter of molds.
Line the floor of the mold with plywood sheeting to provide a smooth surface to the underside of the slab. If making your slab on grade, this step is not necessary.
Line the slab with rebar at intervals spaced for strength, sitting the bar on chairs to allow the concrete to sit both below and above the rebar, sealing it inside the slab. You are now ready to begin pouring your concrete into the mold.