What makes a useful headcanon
A useful headcanon is small enough to fit into a scene. It does not need to explain a character's whole life. A habit, a fear, a private preference, or a repeated reaction is usually enough.
The best ones feel like they could have been missed in canon. They add a little pressure to a character without forcing them into a new personality.
Using the generator for fanfiction
Start with the generated idea, then make it more specific. If the result says a character keeps something private, decide what the object is, who notices it, and what changes after that.
You can also use a headcanon as a quiet scene between larger plot moments. Those small scenes often make action or romance feel less generic.
How to make the result sound like you
Rewrite any sentence that sounds too clean. Add one concrete object, one awkward reaction, or one line of dialogue. That is usually enough to make the idea feel less machine written.
If a result feels too dramatic, make it more ordinary. Characters are often easier to write when the detail is simple: bad handwriting, a snack they hide, a song they pretend not to like.