1. establish a portfolio, demonstrating ability to write about niche topic
2. network with writers in the same niche
3. create a list of publications, websites & newsletters to follow. Read them regularly.
4. contact editors of those trade publications, websites, and newsletters with introduction and/or article idea suitable for that publication
5. carefully read contracts before signing. Negotiate if necessary.
6. network with experts, conduct professional interviews
7. write good stuff
8. meet deadlines, and generally keep the doors of communication open.
9. be humble and willing to do revisions if requested
10. ask for additional work
11. repeat

There are lots of ways to make money as a freelancer. It's neat to see how other folks are doing it.