Start by laying out and staking the four corners of the shed. Dig 6-in.-deep depressions at each corner and at the center of each long wall. Use a torpedo level to check that the bottom of each hole is level.
Pour about 4 in. of crushed rock into each depression, level it and compact the rock with a hand tamper. If your site is sloped, set and level the blocks on the uphill side first, leaving 4 to 6 in. above grade, then use a 2-ft. level taped to a straight 2 x 4 to determine the height of the downhill blocks.