I think @getsuart wanted to know what do you mean by "collaboration", just a cameo in one of your pages? or a fancomic or fanart about an other comic that you publish? Or do you want somebody else to do a fancomic about yours?
I think for the cameo, most of the time, you just have to send a private message to the creator and say "hey, I like your work, I'd like to use your characters in my comic for a cameo, is it possible?" and if the guy says yes, you can do it and put a link of his comic below your page.
I think it works the same way for any other stuff, like fanarts or fancomics, if you publish it in your own comic, then it's better to ask before you do it, and put a link of the other creator's comic.
However if you just made a fanart or a fancomic and you don't publish it in your own comic, you can just send it on the wall of the creator and say something like "look! I love your work so I made you this !"
If you want other people to do stuff about your own comics, well it will be harder because if someone doesn't know your work or is not a really big fan, they won't draw something for you like that... Maybe you might want to organize a contest with your subscribers or something like that.
