Most gardeners now just lay the weed proof membrane on top of the soil that is already there, once its fairly level. There is no real need to prepare the ground first. Make sure you buy a woven membrane that lets water soak through. Cut holes where you want to plant your shrubs etc and then lay either stone chippings or bark on top of the membrane. They recommend approx 3-4 inches of coverage of bark to stop weeds growing on top of the membrane. For stone chippings I think its slightly less approx 2-3 inches of coverage.