I use an inexpensive black foam core panel to set up my painting surface. I then mask the painting area I want to work on with black masking tape. In this example the painting surface is about 10 x 14. The paper is cut to 16 x 20 so it will fit a standard size frame with a 3 inch mat.
The black surface and masking tape give me a dark value to reference as I work. I now have a light value, my paper, and a dark value to help guide me.