As with all of my vector portraits, I start with an initial base image. I’m going to trace face, eyes, ears, nose and lips with outline. Than add some colors and shadows. key point of flat portarit (and semiflat) is to limit your color palette to 3-4 colors max.
To make face symmetrical, I’m working with only one half of the face, than make a duplicate with Effect|Distort&Transform|Transform..
The hole process takes about 1 hour, but if you’re working with lot of details ( wrinkles ), tracing hair and adjust eyebrows, it would take more than 2 hours.