I got some questions about the process behind low poly portraits, so here it is! Hope it explains something
I guess the first step is the most important one. I try to sketch in very roughly, but with a lot of straights, not worrying about an outline, just about the energy and shape design. It's almost like when you are designing a logo.
Then I extract the shape from the lines, picking interesting angles aiming for an energetic composition.
Locking transparency of the shapes I created enables me to work inside of them with a soft brush, getting the form right. I use different blending modes on the brush (Multiply, Overlay, Color Dodge, Soft Light) to get nice color richness.
Then I paint on top, grabbing polygonal lasso tool and creating triangular shapes in strategic places, filling them in with a flat color. Some postproduction (Color Lookup Adjustment Layer - with a preset Crisp Winter).
I do use reference for these portraits as it's mainly a stylization training for me.