I started by talking to my friends and networking with people who needed developers, using online sites to find people hiring or seeking developers I could pitch on my services(this works amazingly well), and doing content marketing to get some SEO and exposure for myself and services. I started with a rate that was a little more than my salary when I left to cover things like savings, retirements, insurance, taxes etc that my job paid and raised my rates any time I had to much work to do. See my other response in the thread for more details.