You know I've been wondering this lately too. I haven't done any freelance work but I want to get into it. I'm still not sure the answer, but I started by turning on the 'Hire Me' buttons on my Codepen and GitHub accounts. I just launched a portfolio website (last night actually) that showcases a few of my projects and lists my contact information. I think my next step would be to check out sites like elance etc. But I'd love to hear other ideas from successful freelancers.