Hi John Your first layer has to be undisturbed soil. If you're doing this next to a house, its likely the soild has been disturbed (original excavation). If the soil is undisturbed, then you can put down p-gravel and compact or sand and wet down well so it compacts to its maximum. You don't need to do both. The purpose of this is to prevent moisture from wicking up into the slab from the earth. Then put down your vapor barrier and then your rigid insulation and then pour over that. Some use builder's mesh (reinforcing screen) in the slab, but this, IMO, isn't necessary if you have preped your surface correctly. A small spary of moisture over the rigid insulation just prior to the pour will keep it from drawing moisture from the just poured concrete...although this amount should be small.