When I start a portrait, or other drawing, the first step is sketching out the outline. I then focus on a section of the drawing at a time, building up the detail in that part until I'm about 80% happy. The final 20% of work comes at the end, once all the detail is in, and then I spend time fine tuning, tweaking and generally making sure each part of the drawing is perfect!
I like to take a photo of my progress at the end of each day on my iPhone. I've used the photos from my portrait of Chester to put together a visual so you can see how the portrait takes shape, from the initial photograph to the final finished piece.
This photo of Chester was amazing to work from, it was great quality which enabled me to really get the details of his features and expression just right.