When you look at your painting subject with an eye toward where to begin, it is tremendously helpful to be able to see right through the darks and middle values to the lightest tones. The first layer of your painting usually comprises pale washes that describe a very general version of the major shapes.
Ignoring the darks can be tricky. To get that dark layer to hold still long enough for you to "peel it back", it may help to practice seeing it as a separate collection of shapes.
Find a high-contrast image, or use one of those below, and make a simple painting of the pattern made by only the darks. Use a single color, and make all the darks very dark. If a shape seems neither dark nor middle value, decide which it is closer to, and round it up or down. We're simplifying here, so some compromise is required.
To go one step further, make a study of the middle values. Remember the progression for the green glass cup? In a similar way, paint the whole page with a middle value, except the areas that are closer to white than middle. Leave them white.
If the two studies could be superimposed, you'd have a 3-value monochrome version of the scene.