Spread 4 inches of washed gravel on top of the prepared subgrade to the extent of the slab. Place a layer of the 10-mil poly over the gravel, overlapping joints by at least 12 inches.
Measure or calculate the thickness of the slab, plus the thickness of rigid insulation. Lay out the line on the insulation with a straight edge. Score the surface of the rigid insulation with the knife held against the straight edge, and snap the board.
Install the board on edge against the wall or footing. Continue along the perimeter of the foundation, ensuring joints are tight and the insulation fits snugly around service entrances.
Lay rigid insulation horizontally against the vertical insulation, and extend under the slab for 24 inches. Make sure the insulation fully contacts the washed gravel. Follow the vertical insulation around the perimeter, scoring and snapping to fit as necessary.
Spread a minimum of 1 inch of sand -- or as deep as recommended -- on the rigid insulation as a protective base for the slab.