When you can see a subject as a series of layers, the job of translating it into paint is mostly done. Remember that in addition to progressing from light to dark, a watercolor also goes from general to specific. For homework, use the photo you started working on in class, or find a relatively simple object in your home. A seashell would be fine, or a teapot. A bicycle probably would not be a great choice. Something that can be seen as just a few shapes is good for this project.
Shape by shape, start by asking, "Is there a way I can paint the whole shape with an overall wash that can underlie everything that will come later?" This will usually be the lightest, most general layer.
Mix up more than enough paint to make the first layer, but before you apply it, ask, "Is there anything I need to reserve?" If there are shapes to save as white paper, draw them in pencil and paint around them.
Make the first layer, then do the same thing on two additional pieces of paper. One of the three will not get any more layers. We are aiming to have a step-by-step display when the project is finished, one painting of just the first layer, another of the first and second layers, and a third of all three layers.
Now look for the middle value strokes that can be applied on top of the first layer. You may want to put them down while the first layer is still wet. Leave one of your first layer pages as it is. That will be the step one illustration. Apply the second layer on the other two.
Now look for any dark strokes that should go on top of the lights and middle values. Paint them onto one of the two layer pages. You should now have three pages that together illustrate your three-layer process.
Bring all three pages in to put up on the homework wall.