I've played around using this technique in PS and this is what i got so far.
Basically what i've done is:
- Extract the subject from the photography, just using quick selection will do the trick, since this selection is gonna be used only as a guide.
- Desaturate the imagen by going to Image>Adjustments>desaturate.
- add an adjustments layer "Levels" to increase black and white tones, play whit sliders until you get a well defined white and black areas "edges".
- Once you got your image composed by black and white mostly, go to Filters>Artistic>Cut out, once again play whit sliders:
*Set Number of levels to 2 or 3 maximum, that will give you a nice definition whit out loosing basic details on your subject, also will be easy to color correct later on.
*Set edge simplicity 3 or maximum 4, this will reduce drastically the jagged edges.
* Set edge fidelity to 1, also remember the the final image will be just a stencil so you will not need too much details
- Now come the fun part, at this point you will have a nice vector kind defined image, whit 3 or 4 color shades, mostly black,dark grey, light grey and white.
From now on you will need to replace those gray shades, either way whit black or white, for this part you will be using your brush tool set to 100% opacity and a 100% flow, also make sure you got your hard edge brush on.
Just paint some areas that you want to add detail like eyes, nose, mouth and ears, jut let your inner artist come out and set him free.
this requires a bit of painter and drawing instinct but don't be afraid to experiment whit those tools, is a lot of fun, believe me.