Everyone does portraits differently. I've been painting and drawing them the traditional way for a good number of years. But Artrage makes it easy to do them digitally. Here is my method.
Step 1, find a reference picture and use the trace mode and the pencil to gently trace details like the eyes, nose, mouth, hair etc. This is important for placement, and makes it super easy so you can concentrate on shading and blending. Then clear the trace and open the picture for a reference.