Find an image that has a wide range of values . Use one of your owm or one of those that follow
Reduce your scene or photo to three values. Desinate each shape as either white, middle or dark. Work in monochrome, with a color that can get dark enough to represent black (Violet, Pthalo green or blue, Indanthrone blue, Burnt Umber),
Starting with a middle value, paint everything except the very light shapes, which will stay white. You may have to round areas up or down and commit to calling them either white or middle.
Next, paint all the strong darks fully dark
That’s all. Take note of where you want more subtlety . In the scene below, for example, the wall facing the viewer really wants two values to keep the door from getting lost in the dark. Don't skip the assessment, it's the most important part!
Now expand your palette and paint a picture, using what the study taught you.