Look for an image in which the major shapes are apparent. These are the shapes that must be separated from each other for the illusion of space to be effective.
Make a drawing of just the outlines of the shapes (no texture, no detail). Assign one of 3 values to each shape: white, middle gray, or black.
Now paint a monochrome study as simply as you can. It will probably be necessary to round your values up or down to avaoid using more than three. Make your decisions according to what you think will enhance the illusion of space.
If you've already done this in class, and you plan to work with the same image, feel free to skip this step.
The finished study, overly simple though it is, will serve you well as a "map" of what must remain obvious in the proper painting that it informs. As you develop a painting from the study, check often to make sure that the basic value pattern is similar to the monochrome version. Squinting helps.