Have some skill where you can do X and it will cause money to happen for the client.
Use existing knowledge of some narrowly specific type of business to give you intimate knowledge of the unique concerns those people have, how they talk about it, and what boundary conditions are going to affect which solutions do and don't fit well (e.g. gourmet pizza joints near college campuses, or heritage language schools to steal a previous patio11 example).
If you don't have that sort of knowledge already, talk to people until you do. If you do have that sort of knowledge already, keep talking to people anyway because you still have plenty more to learn.
Use knowledge of this specific customer to find, approach, and pitch clients who are a good match for your target customer. If you don't know how to find them, you don't know them yet.
Then, pitch them:
"You can have me do X which will cause money to happen for you."
"I'm aware that you have concern_a and concern_b, which I will handle using specific_approach, whereas generic_person_235089 doesn't even have a clue about these important_issues."
LISTEN to what they say, then use it to refine your ability to select potential clients and to improve your future pitches. Easy, easy, easy hack: remember the words they use to describe things and use EXACTLY those words when pitching them.
Run 10 pitches, review, repeat with another 10 target customers. Make money.
Then, go read tptacek's guide to scaling your practice:
https://news.ycombinator.com/item?id=4247615
P.S. It's okay to be a generalist on the back end. It's not okay to be a generalist when you pitch. Address a clear, specific problem for a clear, specific target customer using clear, specific language which shows sensitivity to the issues as THEY see it and talk about it.
98% of people pitch by talking about things that are really their problems, not the client's problems. Actually, saying 2% of people clear that bar is probably being too generous. If you aren't sure, a good trick is to ONLY use language that you've heard come out of a customer's mouth.