The first actual physical effort was to dig a hole about 5 feet deep, about 3 feet in diameter. I can't believe I forgot to take a picture before we filled it with gravel, but here's our hole after the fact:
The tarp is covering a mound of dirt and clay, which is what used to be inside the hole! We covered the pile because the clay was getting baked in the sun, and we needed to keep it moist and pliable so we could work with it.
By the way, it doesn't look like it, but that is a LOT of gravel.