When plotting down the animation, the first and most important thing you'll be putting down are the keyframes. Keyframes are the absolute extremes of motions. A quick example would be a punch. One keyframe would be the arm pulled all the way back, the next would be the fully stretched out punch.
There's no limit to the amount of keyframes, you use as many as you need to make the motion. While a motion may seem simple and quick, it might need many keys since it's made up of many parts. Using the punch example again, the motion can be broken down into 2 parts: The windup and the punch. A more complex motion will need more.
In the case of John's f+H here, it too is made up of 2 parts: the windup and the swing. Thinking about the motions, I figured out we'll need 4 keyframes:
1. His idle pose. This is because it has to start from neutral.
2. The furthest he swings his hammer back.
3. The hammer slammed down.
4. Back to the idle pose, since you have to return to neutral.
Once you've got the keys down, it's a matter of playing with the timing and seeing how spaced apart they should be to make the animation feel right. Basically watch it looping and add more or less frames between keys until you're happy with the timing. Here's what I've got:
The "rough" layer is the one to look at, the rest are just guides. Some will be covered in the Inbetweens section. Also note that the timing here is subject to change. When it gets to the inbetweening stage you might add or subtract frames since things will look different with art inbetween.
I think that's all I need to cover for keyframes as it's pretty straight forward. Here's the completed, keyframed animation:
