It is hard to find good lessons for inbetweening and clean-up. Scott Petersen is an animator who has worked for Disney, Dreamworks and Warner Bros. Animation on projects including The Prince of Egypt, The Iron Giant, Space Jam and Mary Poppins Returns.
In this video, Scott teaches inbetweening in the old fashioned way drawing on paper. Though it’s on paper, the same principles apply. Inbetweening is not just using a light table to draw between the lines, it requires the artist to think about arcs, form, and movement. The flipping and rolling techniques he uses can be recreated in most standard animation software.