I found the routes to my transit system on transit.land, then exported the route and stop geojson files. I converted the route file to Gpx so I can change the line style and see it better. Then I used the public transport plugin to help add info to each stop. It was pretty repetitive, and I'm not even finished yet. I still need to add the actual route and add both route and stops to a relation.
That sounds fairly complicated, so I'll make sure that I've done my research and practiced before I try to add the routes. I didn't know about transit.land, so that's really helpful, thanks :)
If you're adding from an external source make sure you've checked the licence is OK, not everything that's "open data" is as permissive as OSM requires.