Pick 3-4 people who are interesting to you and make as many sketches and studies as you can—quick doodles, style explorations, etc. Stage your subject so that it's not just a portrait, but a clear representation of who they are and what they do. This doesn't mean that a portrait of an painter should be holding their tool of choice—try to come up with subtler ways of conveying their inner and outer personality. It can be a matter of style alone, color choices, composition, etc.
For the next session pick one of the people and bring at least three different sketches for the portrait, as well as all your exploratory drawings.
Refine your sketch if needed and start working on the final. Remember that style is at least as important as the concept, and allow yourself room to make more tests and sketches. You can make separate sketches for likeness, for composition, and for the atmosphere, since keeping all aspects of illustration in mind at once can be a bit much.
Format: 4x6"