Stake the footprint, strip the topsoil and organics, then build the pad up in 6 to 8 inch lifts of moisture-conditioned soil, each compacted before the next, commonly to a 95 percent Proctor spec, and proof roll the result. The pad hands off at the exact elevation and dimension the foundation plan calls for.
More detail
A building pad is manufactured ground, and the manufacturing steps are what separate it from a pile of dirt. Layout first: the pad footprint (typically the building plus several feet of apron) and its design elevation come off the site plan, set high enough that the finished floor sheds water in every direction. Strip: topsoil, roots, and organics leave the footprint entirely, because organic material under a pad decomposes and settles on its own schedule, usually after the slab is on top of it. Build in lifts: fill goes down in 6 to 8 inch layers, never in one push, and each lift is compacted before the next arrives. On the clay side of the corridor, moisture conditioning is the make-or-break step: expansive clay compacts only in a narrow moisture window, so the crew wets or dries and reworks the soil until it is in that window, lift by lift. Where the foundation design specifies it, imported select fill caps the native clay because imported material is controllable in a way native soil is not. Verify: engineered projects get density tests against the spec (95 percent of standard Proctor is the common number), and every pad gets a proof roll, a loaded truck crossing the surface while eyes watch for pumping or rutting. Protect: a finished pad is crowned to shed rain and kept sealed until the foundation crew takes over, because one soaking storm can undo a week of compaction on unprotected clay.