Storyboards. Time lines. Basic Math.
Let's say you have a scene of head moving from profile to full face and rising it's left brown. So you have storyboard with the profile with an arrow showing in which direction it moves. Underneath this picture it's timestamped. Let's say 10'55. Next picture it's full frontal face with timestamp 10'53.
So a between those two frames we have 3 second. 12 frames a second give us 36 frames.
The animators create a crude drawings (photos/renders) of those 36 frames. Especially for deciding where to make smudges because smudges save a LOOOOOT of time.
To make things easier (and production more time and money consuming) each character on screen can have it's own animator that supervise it's movement on the timeline.
Next you have skeletons on which you build. Their joint often have "clicking" ability. So to have full arm motion from curled to straight it takes 15 clicks. 15 changes of position. Easier to count and connect defined position it should be in desired time.
And here where timeline goes into extra play. You have second divided on frames. In some of those frames you animator put extra info like "use hand number2" and 4 frames later "use hand number1". Like with mouth, to say "Lola" you need a certain amount of time. During which you open mouth in particular way. So you can transcribe mouth movement on a timeline and pinpoint certain shape to exact "frame" it should fall onto.
Look at some Aardman Studios making off's. They have full suitcase of Wallace mouth movement. Video and photos from Pirates as reference
In this case you would have pre-recorded speech which then you would transcribe, and attribute number (that is connected to sound) to certain start and duration time.