A base for a shed has two critical dimensions. As a platform, it must be solid enough to support the shed and all its contents. Also, it must be absolutely square. All corners must be at 90-degree angles. If the base is not square, the chances of the shed being built or remaining square are not good. The type and shape of the shed and the base or foundation do not affect the squaring of the base.
Lay out the shed base with stakes and builder's twine stretched between the stakes. Drive the stakes in the ground at the appropriate distances for the shed outline, say 8 by 10 feet. Put stakes 8 feet apart one direction, 10 feet apart the other, in a rectangle with the corners basically square.
Measure from one corner to the opposite corner with a tape measure and record the distance. Then measure the other diagonal, between the other two corners. If the two distances are the same, the shed base will be square, with 90-degree corners. Move the stakes and twine as needed until the two diagonals are exactly the same.
Use 2-by-4-inch boards as an alternate but similar squaring method. Nail the boards at corners into a square or rectangle the size of the shed base. Use a framing square to get the corners as square as possible. Position that form on the ground where the shed base will be built. Measure the form's diagonals and adjust the boards until the measurements are the same, then stake that form into place as a guide for the shed base.