This exercise was adapted from a lesson from Virtual Art Academy (link), Virtual Art Academy is an on line class that I have been enrolled in and highly recommend.
I painted this example in a variety of colors to make it easier to follow. Draw simple geometric forms to represent the masses of the tree foliage (For this exercise, we drew three circles). Mark the source of light. (Here it is coming from the upper left). Sketch in the form shadow shape (remember the shape is concave toward the light).
Get out your value scales to continue. Paint the areas in light a value 6. Paint the shadow areas a value 4. I included the values of the three colors I am using: value 6 (light) to the left.
Add some of the light color into the shadow area: larger strokes in the center of the form where the leaves are closer to the viewer. Smaller strokes are at the top and the bottom (as the form moves AWAY from the viewer). Add some dark color into the light area using the sizes of strokes noted.
Paint the edge of the light area a half value darker than the light area (you painted the light area a value 6, so paint the edge plane a value 5 1/2). If this were round still life object, the edge plane, as the form moves away from you is less lit, so slightly darker. In this case, the explanation given by Virtual Art Academy is that at the edge of the tree form, more of the leaves are seen end on, so the shadow side of the leaves affects the edge plane color.