Learn any new skill by practicing it in public
Example like ad copy, visuals.
Decide what services you are going to offer to your clients, and how much you will charge for it.
Charge on hourly or per projact basis.
Plan out terms you will include in your contact like number of revisions, byline
don't forget to....
Use LinkedIn or Twitter to find clients, send them personalized message.
I am working as a freelance writer and marketer for 3+ years.