Choose a simple still life object. An onion works, or a mango. You can look around the house for something with a wide range of values (the kitchen is a good place to start), and a simple form. Keep the lighting simple. Sidelight works well. A cast shadow helps create three dimensionality.
Begin with a monochrome value study, in which the subject is radically over-simplified. Where do you need more subtlety? More specificity?
Paint the subject a few times, working toward a version that only includes the most telling strokes. After you've painted several versions, try painting one from memory. Please bring all of your studies to class.