I’ve found that one of the first things you have to do when you decide to become a freelance editor (please don’t make me say “hang out your shingle”; I strongly dislike that expression) is tell your friends.
Your friends can often become your first clients. One of my first clients was the husband of one of my best friends. He and a colleague needed to make some corrections to a paper they wrote in order to make it compliant with APA style. Since he disliked doing something so tedious, my friend recommended that he have me do it. I love making corrections to anything, no matter how tedious, so I accepted the job.
Another of my best friends also needed something of hers edited, and immediately thought of me. I edited her paper as well.
Yet another good friend has hired me for personal projects, as well as material for the church which she pastors.
Of course, you can’t count on your friends to be your only clients–you definitely need to reach out! But they can be a good starting point.