You can nominate up to four space-related fandoms, and up to four characters per fandom. (We cut it back from six because we're hoping that will improve our odds of matching people up. The minimum for both requests and offers is three, so you won't be forced to request anything you don't want or offer anything you're not comfortable with.)
Please note that while we will definitely be matching on fandom and media, and will try to match on character, the final results are going to depend on what the sign-ups look like. But go ahead and nominate characters, and we'll see what we can do.
You'll need an AO3 account to participate. If you don't currently have an AO3 account, you can request one here. The waiting period is pretty short right now, so you should be able to get an account in plenty of time to nominate. If you run into problems, please let us know.
Nominations are open until December 10th.