Pretty easy.

s t e p - o n e :

Open Pixlr and your image of the person. You probably don't want it to be zoomed in on their face. Go for about half body - full body. I also like to look for paintings of people. In my case, I chose a painting of Nina Dobrev.

s t e p - t w o :

Click on the Eyedropper tool. This is right next to the Text tool.

s t e p - t h r e e :

Click an area on the face. I try to find the darker spots, but not the darkest. Click around until you find a good color to color in the face.

s t e p - f o u r :

Click on the Brush tool. Make sure it is the color you've just selected. I used a softer brush.

s t e p - f i v e :

Color in the face. Make sure to stay in the lines.

That's really it! The hardest part is just getting the right color.