I was a full time freelancer for few months. Even now, I work remotely, so here is my take.
I will divide your question into 2 halves.
- Do you know SEO / SMO completely? (in and out)
If yes, you can proceed to 2nd step.
If not I suggest reading some great blogs
2. How to get clients?
PS: This may not be a step by step guide, but will give a head start.