Hullo, it's me, yet again. This time, I am here to advise you what to do when writing a side-story, and how to proceed afterwards. This is a very simplistic guide and may take very little time, but I'm doing it anyways.

To start off...
Always contact the original author if its a verse you've recently jumped into. [TCB and Fallout don't count with their number of spin-offs.] This can help you get access to lore you might have otherwise not known and thus help your story in joining. Not only that, but it really helps when you run the idea by the original author, because, if they like it, their readers likely will as well!

Your first chapter in all its glory...
While the idea may be sound, execution is more important in the long run. Make sure you have a link somewhere, preferably on the main description, to the original story. This helps avoid people claiming you're plagiarizing and thus getting those nasty downvotes. Execution can actually end up changing people's minds too! They could hate it at first, but how you continue can really affect someone's opinion.

Author's word is law...
I have dealt with issue in which I, as the main author, was ignored and thus had to deal with some troublesome things I hadn't factored in. You should always ask questions about instigating anything overly new, otherwise, you could end up in a bit of trouble with the author.

That's it... for now.