Here is a great starting point for making clay wall material.

2 parts sand (mason or graded sands but have a few different sizes 70-20-50 meshes)
1 part clay soil (screened through 1/8 to 1/4 screen to get the bigger bits of stuff out)
3/4 to 1 part chopped and screened straw or recycled saw dust from the job

Spread a sample as big as you can on the surface you intend to work on to see if this is too dusty (not enough clay) or cracks easy (too much clay)

You can apply such a mix up to 1/2 thick, it will cover lots of irregular surfaces. You can change the mix as you wish or add natural pigments. I like the colors from

It is really fun to explore the best application for the specific wall at hand. It changes day to day. And these materials are so forgiving! Eco Brooklyn is a professional clay wall applicator for Brooklyn brownstones, but truth be told anyone with the time and interest can do it.