Take the rough sketch for your first in-between – again, not the first one in the sequence, but the first one that you drew, bridging the halfway point between keys – and align it on top of your two keys. You should be able to see both keys through the sheet, as well as your rough motion sketch of the in-between. The same way that you marked halfway points between major areas on the roughs, you’ll do again with the detail – finding things like the eyebrow, tip of the nose, mouth, shoulder, etc. and sketching lines between them before marking a halfway point so you know where they should be on the key.
Doing in-betweens on detail is something that takes a lot of practice; even with the little sketched guides, a lot of what you end up drawing is estimated and eyeballed. You get the hang of it after a while, although if you stop doing it for any length of time, it’s quite easy to fall out of practice.
