Do it slowly at first, as you progress you'll begin to do it instinctively. There are people working in the industry who aren't smart enough to chew gum and fart at the same time, and they can do it. So don't worry. It takes a little time, but you'll get the hang of it. These are the drawings that go between the keys in order to make the action look smooth.
All frames are equal, but some frames are more equal than others.
You must determine which is the most important frame to draw next. In this case, the most important is frame 6. It is the most stretched frame in the falling sequence. Therefore I would consider this almost as much a key frame as 1 and 7. If you have a backlight, switch it on.