We will start with the image we want to edit, and we'll create a new layer. This is the image we are going to use in this demonstration:
Now we want to forget about that for a minute and open a new document, around 150px by 150px. Create a new layer and with a 1px hard-edge brush, paint some lines like so:
Now press Ctrl+A to select all, and goto Edit > Define Brush Preset. Give it a name if you want, and press Ok:
Now close that document and go back to our original. Select the Brush tool and choose the brush we just created from the drop-down. Now click on the Brushes Expanded view:
Click on Shape Dynamics, and set the following (it is advised that you alter and play around with these):
Now we have set the brush, we can begin painting. Choose your color and also brush size - our image is relatively small so we're going to use around 10px. You will probably find that you'll have to change this for different areas, though. For instance, in tight areas like a moustache, you'd need a smaller brush.
Once we have our color and brush size selected, create a new layer and start painting:
You may find it useful to change the facial hair layer's Blend Mode to Multiply. Using different layers and opacity settings, you can be very creative and have some fun: