Drive a form nail through the stake and into the form material, holding the top of the form at the desired elevation for the top of the slab.
Hold the plywood forms against the stakes, adjust to the correct height, and fasten to the stakes using deck screws or form nails installed from the inside of the form through the stake.
Beginning with the bottom riser, nail the riser form in position using form nails driven through the outside of the side forms.