On this project, I just started out with a single layer of cardboard where I glued down the bottom layer of 1"x0.5"x0.5" Styrofoam bricks with Gorilla Glue. All other rows I just used white glue.
I had a few squirts of spray foam left over after some gap-filling in my house, so I just used that for some thickness, then glued down random bricks cut on angles for that collapsed structure feel. After drying I added my favorite terrain standby of spackling compound for added filler and then some sand where the grass has been worn away. I also attacked the bricks with a small cautery to put some crumbly/ancient touches to them.
Base spray-coat black, ftw!
Hit the stones with a thick dry-brush of a medium gray, then a light dry brush of a light gray.
Started adding the flock. I've had the most luck doing small separate sections as it doesn't lead to big balls of fuzzy green goop.
And the finished piece, ready for the characters to discover. A campsite of travelling murderous cultists? A hidden trap door to a Mega Dungeon? Or a defensible position when the Spider hordes attack? Perhaps all three...