The ground on which a shed is built does not have to be perfectly level. The foundation for that shed must be. A shed should not be situated, however, on a very steep slope, which would be subject to heavy rain runoff and erosion, unless you plan to build poured concrete stub walls such as are used on house foundations. A more typical shed foundation can be built on unlevel ground using concrete blocks.
Determine the angle of the slope. Stake out the basic dimensions of the shed. Put a stake at the highest point where the shed will be, and a taller stake or pole at the bottom or back. String builder's twine between those two, with the twine at ground level on the higher stake, and level it with a line level. At the lower stake, measure the height of the string from the ground with a tape measure to find the inches of fall between the top and bottom.
Use that measurement to figure how many and what size concrete blocks to buy. Plan to set one concrete block pier at each corner of the shed, in the middle of each wall and in the very center of the shed. Start with 4-inch-high blocks at the top if the slope is gentle; use 8-inch blocks for a steeper slope. Combine 2-, 4- and 8-inch thick blocks to get the proper height.
Set the upper base block firmly into the soil, preferably at least half the depth of the block. Dig out the dirt with a spade to place the three upper blocks -- at each corner and in the center. Use a long board and a 4-foot level to get the tops of those blocks level across the width of the shed and also lengthwise by adjusting the dirt base.
Make center piers by stacking blocks to the height of the top of the first block piers. Combine an 8-inch block and a 4-inch block, for instance, if the fall between the top of the slope and the center is 4 inches. Do the same to make the block piers at the back of the shed. Set a long 2-by-4-inch board across the blocks with a level on it and adjust the blocks until the tops are level the length, width and diagonals of the shed.
Make sure the concrete block piers form a square foundation. Measure from corner to corner and adjust blocks until the diagonal dimensions are identical. Place full blocks with openings up; that is the strongest dimension. Fill those openings with concrete as an option to add strength to the piers. Put 2-inch or 4-inch solid blocks on top of the piers for a more solid foundation for a floor frame.