Mark the ground where the walkway is being installed with spray paint.
Excavate the area inside the paint, digging beneath the frost line. (Your local zoning board can tell you how deep this is.)
Smooth the soil inside the hole with a rake and tamp it down.
Place expansion strips against any edge the walkway comes into contact with, such as a patio or the edge of the driveway. This allows for the concrete to expand and contract.
Drive wooden stakes into the ground along the edge of the trench.
Nail 2-by-4 boards to the wooden stakes along the trench (these are the sides of the form).