What to do before laying topsoil?

Could you please elaborate on the steps one should take before laying topsoil? Is it essential to test the soil's pH levels and nutrient content beforehand? Should any weeds or debris be removed from the area? Additionally, is it recommended to add organic matter or fertilizers to improve the soil's quality before applying the topsoil? Lastly, how crucial is it to ensure the area is level and compacted to avoid future problems with drainage and erosion?