The first step was to build up a wall of cardboard over a set of blueprints I had drawn. These were set into clay and, as photographed, vasalined to prevent it sticking to the plaster.
I then poured plaster into the cast alowing it to flatten on the surface so I got a flat surface.
When the plaster became solid, I removed the walls and took the clay base off. I then sanded at the surface of the plaster to get a smooth wall and got this finished result: