The easiest way to build a shed on uneven ground is to start with a foundation of concrete blocks. To compensate for the ground's slope, you simply stack up additional blocks as needed on the low side of the site to create a level foundation. You lay the blocks in two or more straight rows, spacing them a couple of feet apart within each row. Align the rows so they are parallel to the long sides of the shed, with the two outer rows positioned to support the rim joists of the floor frame. Be sure to use solid concrete blocks, which come in 2-inch and 4-inch thicknesses and measure 8 inches wide by 16 inches long. Do not use standard wall block or any other type of block with hollows or open cavities.
Lay out the perimeter of the foundation, using stakes and mason's string. Measure diagonally between opposing corners to make sure the layout is square (has perfect 90-degree corners); the layout is square when the measurements are equal.
Mark along the strings to represent the location of each block, spacing the blocks about 24 inches apart. For a standard 8 x 10-foot or 8 x 12-foot shed, two rows of four blocks (with one row along each long side edge of the shed) is sufficient. For a larger shed, you may need one or more additional rows spaced evenly in between the side rows.
Dig out a level patch of ground for each block location to a depth of 2 to 3 inches. Tamp the soil with a hand tamp. Add 2 to 3 inches of gravel to each patch, and spread the gravel smooth. Tamp the gravel so it is perfectly flat and level.
Position a 4-inch-thick block onto each gravel bed, using the layout strings for proper alignment. You will need only one block at each location on the high end of the site. Stack up 4-inch blocks on the low end until you approach the level of the high end. Set a long, straight 2 x 4 to span across the blocks at the low and high ends, then set a level on top of the board.
Fine-tune the height of the low-end block stack using a 2-inch block at the top of the stack, if applicable, and/or shims made from strips of asphalt shingles. When the high and low blocks are level with each other, complete the intermediate stacks as needed so all points in the row are level with one another. Repeat the process with the remaining rows. Make sure all stacks in each row are level with one another and that the rows are level with each other.
Disassemble each stack, then rebuild it, using construction adhesive to glue the blocks together.