For the last scene of my show I made a simple stonewall using garbage. The modellers often build from Depron foam (it's a closed cell styrofoam) for its lightness and it's very easy to work with it. I didn't bought a Depron sheet but washed up a Depron tray of a shrink-wrapped food, then cut the rims off. You can draw the stone pattern onto the foam by hand or using a printed pattern and carbon paper. For inspiration you can find many wall pattern on the net. After drawing carve the grout lines out of the foam. For this I used a needle file.
After carving you get a very real looking grouting and too smooth stone surface, so you had to damage the stones in some way. I used acryl based varnish spray for this. This varnish corrodes the depron. I sprayed the stones with this varnish then pressed over with a toothbrush. If you need deeper holes, spray some varnish into a bowl and dip a cotton bud into it, then press the bud into the depron.
Now you have a very good texture. The following step is the painting. The colors depend on the type of the stones you choose. I made a sandstone wall, so my base color was sand yellow. I used acrylic paint. After the base color you can rich the wall other tones of the same color by dry brushing.
Now you have to fill the grout lines with something, I used dry tile cement for this. Simply spoon some cement onto the wall then sweep it into the gaps with a brush. Then you can fix it with varnish.
I used some grey paint on the lower part of the wall imitating the splashing mud, and some green for mossing. It's still not ready, I think It needs some more painting, but the most part of the work is ready.
I found ideas for this project in a modelling book written by a hungarian man, László Adóba. This book was published in english, too. I got the hungarian version as a Christmas present from my husband. It has many great tricks for making miniature scenes we can use.