So my kid's high school is doing a production of Man of la Mancha. Much of this play takes place in a Spanish Inquisition (I bet you weren't expecting that) prison. Speaking of not expecting things, the theater director contacted me ten days (!) before opening night and said she needed some dungeon walls. I would make a comment about the flakiness of arty types, but I also happen to be in that category so I probably should keep quiet.
Fortunately, I already had a technique for creating what she wanted that I developed for doing my Halloween displays, shown here. I decided that while I was working on this that I would finally get around to documenting my process and sharing it with the Instructable community. You can use it for creating stone walls, stone blocks, or bricks.
The main material you will need is some extruded polystyrene foam. This kind of foam is a solid sheet, not like Styrofoam, which is made of lots of tiny foam balls compressed together. Styrofoam might work in a pinch, but it is much more fragile and probably won't give good results. The extruded polystyrene is sold as an insulating sheathing material and can be found at both Lowes (https://www.lowes.com/pd/Kingspan-Insulation-R5-Un...) as well as Home Depot (http://www.homedepot.com/p/Owens-Corning-FOAMULAR-...). It comes in a variety of lengths, widths, and thicknesses, so you should be able to find something that is approximately the right size for your project. Also note that if you need something thicker that what you can buy, you can easily glue several sheets together.
I was asked two cover 15 1/2 feet in width, and just under four feet in height, so I used two 4x8' 1/2" sheets for this project. Typically I will use the bluish-green (Dow?) foam from Lowes if I'm making a rock wall, and the hot pink (Owens-Corning) foam from Home Depot if I'm making a brick wall. Why? Inevitably, these will get a scratch or ding in them so I pick the color that will look the least bad if it get exposed. So I suppose if you were doing a hot pink fairy castle wall (I don't judge) the Owen-Corning foam would be the best choice.
Which brings us to glue. Note that may glues will not only not join foam to other pieces of foam or your backing material, but will actually dissolve the foam. This is a BAD thing. Additionally, nails screws and other mechanical fasteners will not work because they will pull right through the foam (although I will use nails as temporary fasteners for gluing). What I've found that works really well is Liquid Nails Heavy Duty Indoor/Outdoor Construction Adhesive (blue label).
Another thing you will need is some kind of backing to hold the foam rigid and allow you to use the aforementioned nails, screws and mechanical fasteners. In the mausoleum and gate I made in the past, I built a frame out of 1x3s. For this project, I used a sheet of 1/4" plywood. Either one will work and it mostly is the application that dictates what sort of backing I use.
The last thing you will need is paint. You will need a base coat as the primary color of the wall; grey for granite, brick red for brick, etc. This would be the color you would see if you were to squint or look at it from far away with your glasses removed. For this I use an oil based paint because it's much more durable than latex. With the base coat, a flat or matte paint is the best. You will also need a grey paint for the "mortar". If I'm doing a granite wall, I will use a lighter grey than the base paint. The last kind of paint you will need is either "stone" spray paint, or a set of acrylic paints (more on this later).
There are very few tools you will need for this project. I originally used a past-its-prime soldering iron for this, but that eventually gave up the ghost and I found a much better way to do this. First, you will need a propane torch. I like to use a high intensity torch head because I'm impatient instead of the typical "pencil-flame" torch heads. The high intensity ones, however, are more expensive and probably better for someone new at using one to go with the less intense version anyway. Going with the pencil flame will, however take much more time to heat and you will need to reheat more often. As far as fuel, I use camping propane (usually comes in a fat evergreen colored bottle). As far as I can tell, it's the same stuff as what they sell with the torches (propane is propane), but for some reason camping propane is cheaper ¯\_(ツ)_/¯. Also, I tried MAP, which they usually sell with the high intensity torch heads. I found that it was the torch head that made the appreciable difference and MAP should be just be called MEH.
Next you will need a steel rod. probably the best thing to use would be a long 3/4" diameter steel rod with a wooden handle, but since I have no idea where you'd get one of those (unless you made one), I just used a 3/8" carriage bolt. Why? I had it in my junk hardware bin. Best to use something you aren't too attached to since it will be covered with gunk (technical term).
You will also need a magic marker (color unimportant so long as you can see it against the foam) and paint brushes. For the paint brushes I used a 3" and 1" chip brush (this isn't fine furniture), and since I had a lot to cover and not much time to work (less than 10 days!), I also used a paint roller and the standard accouterments.
Since you are going to be using hot burny stuff (technical term) you want to be using stuff that prevents you from getting burned... BADLY. Primarily this involves not getting burned by either the torch or indirectly through the steel rod. Like I said earlier, a long rod with a wooden handle probably would be best, but since I was using a simple carriage bolt, I made due with some welding gloves. At times it got a little unpleasantly toasty, but I never burned myself. You could also use some tongs or pliers, but I'd suggest that you have a way to lock them (e.g. vise-grip pliers) so that the red-hot steel rod doesn't go flying into something that doesn't like red hot steel rods being flung at them (e.g. your leg). It would also be a good idea to have a fire extinguisher nearby and ready to go, just in case.
The other bit of safety kit you will want is a decent respirator. Melting foam throws out some nasty fumes that I can't imagine would be good for your lungs or brain cells.
Okay, boring stuff done... let's get on with it.
How you do this will depend entirely on what effect you are going for. If you are doing bricks or blocks, you will want a regular pattern. If it's a rough stone wall, as I did here, you can take more creative license. Very simply what you do is use your magic marker to draw the negative area (e.g. the mortar) between your blocks, bricks, or stones. Here I just free wheeled it, just to get an idea of what I wanted to do. If I didn't like it, I could just draw a new line or even ignore it entirely when I got to the melting phase. It's just easier to lay this out with a magic marker and easier to change (the paint will cover the marks).
If you are doing regular objects, say bricks, it's much better to have a template you can use. When I do a brick wall, I cut out a brick shape in some scrap Masonite or plywood and trace the edges to draw the bricks in a regular pattern.
If you want to get really fancy (or don't trust your artistic ability), for under $20, you can get one of those paving or driveway concrete molds to use as a stencil. It's up to you how you want to do this. Have fun with it.
One additional aspect I would point out is that if you have a distinct front and back to your project, you should work with the blank side of the foam. I originally thought that it wouldn't matter because the paint would cover the print that the factory puts on one side of the foam. While this is true, the process that puts the print on this side slightly compresses the foam. I once made a fake tombstone before I realized this and if you look at it from a certain angle you can see the pink panther staring out of it. This somewhat takes away from the effect.