Excavate the desired area to a depth of your thickest stone plus 2-3 inches for the gravel base. For example, if your stone is 2 inches at the thickest point, then you’ll need to allow for a 4-5 inch total depth.
You can achieve this one of two ways, or a combination of both: 1) by digging down into the existing grade, or 2) building on top of the existing grade and adding forms or a border along the sides to contain the area.