When a radio interviewer asked Terry Pratchet if he thought his books were "literature", Terry replied something like this.

"Whether you count as literature or not is decided by a vote taken about fifty years after you're dead. I just try to write books that people will enjoy reading."

With folk songs, the process is somewhat similar, although the status of any particular song is established by the people, rather than by the critics. And the vote may sometimes take place while the author is still alive.

Of course, anyone can attempt to write a song "in the folk style".   If they then choose to call the result a "folk song" - well, there's no law that says they can't. But for myself, I think that's a poor decision.

A better approach, in my view, is just trying to write a song that other people will really enjoy singing. If you succeed, and if that song gets sung so often that it becomes part of the shared culture of your community, then it will be a "folk song", by my reckoning. (Though I maintain that it won't be "traditional" until nobody living remembers who wrote it.)

Wassail!