I was struggling to do the same. figured i could use my first post here to talk about it and make sure i can post cross instance.
I wanted to subscribe to lemmy.ml/c/dota2 from kbin.social
i ended up finding that i could access that instance from kbin from the following url:
https://kbin.social/m/dota2@lemmy.ml
maybe this will help you out
the generated images have strangely textured backgrounds. before i realized that it was quite tricky.