If you have good extreme drawings and thought out charts, inbetweening is easy. If you consider that inbetween drawings can outnumber extreme drawings by 3 to 1, the whole process of animation becomes less daunting.
Of course that means you need to have a good animator (the guy/gal who does the extremes and usually the keys), a good layout artists, a good storyboard and a good design, but if you can only have one -it's a tough choice, but a good animator does so many things that's the most important position to fill.
Doug Compton is one of the best. Here's a simple series of drawings to be inbetweened.
Drawing A05 works into drawing A10.
That means there are 4 inbetweens. It's typical to be given 3 inbetweens, but 4 is not unheard of.
The chart or ladder which shows how these 4 drawings are to be made is written on A10.
chart on A10
From this we see that A08 is a direct inbetween of A05 and A10. So that's the next drawing we do.
Doug, being a great animator, has already started it for us.
He's done the broad, important "acting" part of the action and left the rest to be filled in. This drawing is marked "C. D." (complete drawing).
chart 08 "C. D."
Now back to the chart.
Once A08 is drawn, move backwards to A07. This is the direct inbetween of A05 and A08. Then A06 which is again half the distance between A05 and A06.
Next, move forward to A09 which is between A08 and A10.
Note how A11 "pops" from A10. It's not microscopically inbetweened. Because of this, the animator draws it as an extreme -right after A10 which is also an extreme.
There's no formula to when or where extremes fall. There could be 5 in a row, there could be 2 in 24 frames. It all depends on the action.
A11 works into A15. This is a common spacing -three drawings (I'll generally refer to the number of drawings as "space" whereas the number of frames is "time" -although "space" is really the distance between between drawings) -here there's not broken up evenly.
In chart 15 you see that A14 is the direct inbetween of A15 and A11. This spacing builds a natural ease into the animation. A "regular" inbetween would have A13 directly between A15 and A11. Though this may be "regular" and eased inbetween is very common.
So you drawing A14 between A11 and A15, followed by A13 between A11 and your newly created A14, finally A12 between A13 and A11.
Note the X between 14 and 15. Note the height of 14, then 13, then 12. Note the arcs on the bottom. This is how animators communicate with assistants.