There's a tough balancing act at play in the creation of a character portrait, or caricature.
The trick is choosing which features to exaggerate. For example, it might seem that exaggerating the size of the nose is the right thing to do only to realize that actually it's the eyes that need to be smaller and by effect, the nose appears larger.
In the beginning, keep everything loose and free, don't get pulled into the details. Try to see the whole from a distance and try to capture the essence of the subject. This part is made easier by producing a few studies of your subject in preparation.