As you can see from above, I love doing caricatures / portraits using mixed media, and for my own reference, I'm documenting the method I used above for Keira Knightly (as it worked so well).
Stage 1: Pencil sketch on plain paper
Stage 2: Transfer pencil sketch to Bristol Board using a lightbox and Ink pens (in future, I might just use pencil and add ink later - we'll see...)
Stage 3: Water color underpainting of subject
Stage 4: Colour pencil blending and detail layer
Stage 5: Guoache highlights / details
Stage 6: Acrylic background (includes slow dry medium to create a "wash" like effect).
My next attempt will be Christopher Lee / Peter Cushing, at which point I'll document each stage...