The fun thing about faces, especially for a tween girl, is coloring them. It's like doing oodles of makeovers but instead of makeup you get to use paint, crayons, pencils and markers. It's surprising how similiar they are! I wisp of paint under an eye takes the same technique and skill that is required for applying eyeliner under a real eye!
If your passion isn't drawing faces...a project like this could also be drawing a book of flowers, doing a daily still life sketch, painting with a different color of paint on each page, or even turning a scribble into a colorful abstract. The point is to do something you love and study all the ways that you can explore that topic or technique.
Try to keep your workbook/journal under 30 pages long and get going!