Draw the outline of the stones directly onto your Styrofoam sheet. You can use a source image of a stone wall as a guide, or project an image directly onto the sheet for accuracy.
Heat up the wire cutter and use it to slowly sculpt stone shapes into the foam. You can carve lines to represent the mortar in between the stones and shave thin layers off the surface to create round or flat stones. Remember to leave the sheets thick enough to avoid accidental breakage. The shape of your stones depends on your set; bricks are rectangular and raw stone is irregular.
Continue carving stone shapes into the sheet until the entire sheet has the rough appearance of a stone wall.
Cover each stone in a thin layer of clear glue and brush a layer of sand over the surface. This adds texture and realism to the stone.
Coat the walls in gray, brown or red acrylic paint, depending on what type of stone you would like to mimic. Do not use spray paint, as it will melt the Styrofoam.