1. Open a new document in Flash
2. Draw a shape
3. Select another frame (how long you want the tween to be)
4. Right click and select Insert Blank Keyframe
5. Draw a new shape in this new frame
6. Select one of the frames between the keyframes
7. Select Insert > Shape Tween
8. Select the first keyframe and go to Modify > Shape > Add Shape Hint
9. Move the shape hint so it is in the same position at the beginning and end
10. Go to the last keyframe, and move the shape hint to the corresponding position
11. Test using Control > Test Movie