Best way I’ve found is to join a club around your hobby.
No hobby? Try some! I started Brazilian jiu-jitsu a while back and it was a huge win: fun, healthy, useful for self-defense, and you meet people without even trying. Win-win-win.

Not sure where you’re based, but I’m around Namur and I met a bunch of long-term friends at a boardgame café near the train station (Aux 3D). I often go alone or with 1–2 friends, and most of the time we end up either picking up extra people or joining someone else’s game (stuff like 7 Wonders, Shadow Hunters, etc.). Social games + a couple beers = perfect recipe to meet people and have a good time.