Get a 12 inch or larger power auger (put it on a tractor's PTO). After you level the ground to about 4 inches below grade auger down about 30 inches along the edge -- how many holes depends on the size of the perimeter. Punch some holes in the middle if you want -- one in the center could become a dry well for spills.

Put in the edge forms and pour. Stick some bolts in to attach the wall.

Sure glad I'd hire something like that out now-a-days!