While you're all tuned up to see in layers, the time is right to practice making shadows as a layer that goes directly on top of the sunlit light shapes. Hopefully, the weather will offer us an opportunity to work from life, but if not, here are a few vignettes that feature shadows:
If you are not yet in the habit of having a piece of practice paper handy while you paint, this exercise is a good time to establish this essential bit of housekeeping. When you decide on the color and value of the lights that will have shadows cast upon them, make a big patch of the color on the practice paper. Then, when you are mixing the shadow color, you can try a stroke on the practice patch and see immediately if it works.
Don't forget to practice the kind of edge you want, too. Some of these are soft.