First i have stuck two pieces of foam board to gether of the desired shape and size of the wall.I used PVA glue and left over night.

You can use superglue for a quicker procedure

I then mixed up some poly filler. easy sand is best for sculpting.

I coated the foam board with a coat of PVA to help the filler bond to the surface.

I coated the whole piece with a coat of the filler and made random impresions of stones with thicker applications in places

This was then set aside overnight to dry

When the filler was completely dry i started to carve out the individual stones on the wall

make sure on this type of wall you keep things random rather than uniform. Some stones can cover two cources of stonework

observation of the real thing will help.
Take pictures to work from reference.

here you can see the wall completed of the sculpting work