Ed van Dorsten scratch builds very effective ruins and other models from Styrene sheet plastic:
The main material I use is plastic sheets approximately 5mm thick. You can get these from hardware stores or used packing materials. The plastic has to be a little flexible - if you can bend it a little without breaking it it's ok.
Draw the outlines of the walls etc. on the plastic and then you're ready to cut. Cut the plastic at room temperature or you might chip or break the plastic. When cutting keep in mind that you must only apply controllable force or bad things WILL happen. For cutting I use a Stanley knife. I tried a modelling/surgeon knife but that didn't work too well (Bandaid please!). Cut the first cut not to deep, this cut is mainly to lead all following cuts. The plastic will be cut through after 2 or 3 cuts.
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When the wall section is free you can use your knife to add texture and stuff to your wall. Slashes are made by cutting a V-shape in the plastic, hold your knife under a 45 degree angle and cut a line, do the same under -45 degrees. The cut should look like this seen from the side: -----------V------------ Holes are made by rotating the tip of the knife in the plastic. I also rounded the sides of the walls to get a more natural look. It's mainly a process of getting the hang of it.
I glued the corners together with a 90 degree corner profile which you can get from any hardware store. Add some detail stuff, be creative and you're ready to paint.
I sprayed the first layer black, waited for 10 mins and then sprayed on some white and grey. The reason for the 10 min waiting is that the drying first black coat will shrink adding some more texture to your wall.
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Well that's mainly it. This is my first piece this way so things can only get better I guess.