When you get to know someone, you might ask about their interests or hobbies. From that, you can better decide what activity to invite them to join, or on what topic to have them converse, whenever you meet again.

Any interest or hobby appeals to variously many people. If you have the same interest as the person you meet, the commonality informs you much better about how to connect with them. Ceteris paribus, if the interest is more common, you and your interlocutor are more likely to share it, but would bond by it more weakly. Think, for example, of how two extreme ironers connect, versus two people who "like Linux", versus two people who just "like music". If, as typical, you befriend people by overlap of existing hobbies, meeting thoroughly-relatable people relies on much luck, rather than effort. We can do better.

Instead, make or do something unique that lasts, or that at least has effects and evidence that last. Ideally, your creation manifests your thoughts, as from a collection of essays. You and/or someone you meet — whoever has social effort to spend — looks at what the other did. From what you alone made follows a unique hobby — an idiohobby — of any activities which could precede and follow it. One would assume you're enthusiastic about any significant aspect of what you alone made.

If you treat idiohobbies like normal hobbies, you'd need to have made the same thing as the person you meet. The real criterion is a lower bar. You just have to become familiar with what the other person made, and willing to partake in its implications.

A normal person might ask "what do you like to do? Might some of that also be what I like?". I, in search of idiohobbies, will ask "what have you done by which I may know you?".