I haven't personally done any commissions but based on what I've seen other artists do and also based on work I've done for friends, here's how I sum it up:
1. You study anatomy and drawing techniques for ages, gradually improving and growing a fan base
2. You set up a business email (separate to your normal email to better keep track of it) and a paypal account (paypal is easiest to manage and give refunds if things fuck up)
3. You set a bunch of prices for different levels of work (eg. Sketch, outline, coloured, shaded, etc.) And make a post (with your business email address) about it (here or anywhere else that you have a following)
4. NEVER! expect to be asked to do something then just do it. Send the client progress as you go and expect them to ask you to change aspects and be prepared to do so.
Josef-Eliasson
JJ JJ
Greebs
wut