I’d say that step one is dropping the word “freelance”. I’m in Sweden, and more often than not, freelance mean lower pay and short stints. That’s not what I want, and maybe not you either. A title such as Senior WordPress Developer, Something Something Expert etc is already niching it.
Finding a niche is also about deciding on what an interesting project is for you. For me, that’s a kind of product-market fit where my skills makes the product and the market is a segment of customers with needs I would love to work at fulfilling.
Try to look at your skill set in the eyes of a potential customer. Why would they choose you?
My way of niching is mostly by positioning about what I won’t do (projects below 2 weeks, WordPress and PHP - not meant as offensive but it’s my position, marketing pages, design and a few more). Combined with lots of networking I’ve found this to be effective. Most now see to my agency (3 ppl) as only doing advanced web apps and complex projects, which is what we do nowadays.
Best of luck to you!