I did a large mural in a library. The wall were painted white, I painted right on the walls.I did drawing that I wanted on the walls in my sketchbook , then I xeroxed them on a clear transparency sheet and put these on an overhead projector. That way I could easily enlarge them to correct size and proportion.
For paint I used reject house paint from Home Depot. They sold it to me for 1.00 a gallon and .50 for a pint because it was for the community. I mixed colors on throwaway styrofoam plates. Hope this helps you.