Start with one piece and little support. Screw piece to wood 2x4

Screw additional pieces to each other

Gaps that will be filled.

Corner gaps that will be filled with foam

Break edges with pliers

Filling cracks with foam

Fill cracks with foam

Make sure all cracks are filled with foam

After letting foam sit for a few minutes. Punch it down with your fingers.

Now its time to paint

Strong enough to stand on

Strong enough with no support to stand on

1 1/2 hours later finished rock