Flash Frame-By-Frame Animation: Detailed Animation Part I

In-Betweening II

With those points in place, you can use them as a reference to rough-sketch the positioning of the head, neck, and shoulders--but don't rely solely on those. Picture the motion in your mind, think of where it should be at that point--estimate and use your own judgment while sketching. Another purpose of this rough-sketched stage is to let you work out any problems in the motion before you get to the detailed stage, so don't worry too much about making it perfect; you can always scribble over it later.