This is a rough port of my blog post at http://www.artificialworlds.net/blog/2017/01/17/submitting-a-package-to-f-droid/
Feel free to rip out the vanity link and change in any other way.
It's more a narrative than a proper guide, so may either need editing or need renaming. I'm happy to make changes or have someone else edit it.
NicoAlt on IRC told me to make a dir called _guides but I wasn't able to get it to work (even when editing _config.yml) so I've just made a new file in _pages for now.