If you are just removing soil, you should be fine to pour, but if you have to add fill, you need to rent a compacter. Sand also needs to be compacted, if nothing else, use water. Concrete needs to dry slow to add strength. That is one reason for the plastic, the other is a vapor barrier, if you are going to enclose the slab with sides and roof. Water the concrete several times to slow down the cure for about 4 days. We must do our concrete here in Florida differently then what is custom in your area because we always use 6" X 6" mesh.