Often collaborations sort of happen by chancemutual interest ― you start talking with someone, an interesting question comes up, and you decide to pursue it. I don't think I have ever attempted to find a coauthor just for the sake of finding a coauthor (but that doesn't mean you shouldn't do this). Most of the time, it's either with someone I already know really well (this should be easy to initiate!) or with someone who has worked on very similar stuff (which is also not so hard). It's easier if you're generous sharing your own (half or full) ideas.
Just like not all of my own projects end up working out, not every collaboration ends up working out. For example because you get stuck or someone loses interest (is overwhelmed by other work, etc). That's ok; don't beat yourself up or try to force something. But do make an effort.
You might find it easier to be the initiator (coming up with the idea and finding someone suitable), or you might find it easier to join someone else's project. In the latter case you could attend a lot of talks and show interest in other people's work (especially if you are indeed interested!).