After the first wash dries completely - what I call "bone dry", I then mask the next light values to save. Here you can see the masking. It's okay to use masking tape, or frisket film, but I find that they leave funny edges when the masking is removed, so I stick with brushing on the masking fluid.
So the first masking was to save the absolute whites of the paper. This masking application is to save the value 2 areas. I use a value chart from one to six, one being white and six being black. Each number represents a paint application. So, each stage of the painting is masked to save the next layer of color and value.