Scrap materials work fine for making form boxes. The boxes are a few inches deeper than the part of the pier that will appear above grade so that the forms can extend a bit into the ground.
After the boxes are connected by a perimeter of 2-bys, the entire assembly is leveled at once and staked in place.
It takes about 15 to 20 minutes to dig each hole to the required depth. Usually a shovel, a crowbar, and a post-hole digger suffice, but it's a good idea to keep a recip saw and jackhammer handy in case of obstructions like a rock or a root.
For a cantilevered shed, the main floor beams were assembled on top of the form boxes. The anchor bolts were installed before the pour, ensuring perfect bolt placement.