SIGs (Special Interest Groups) are basic building blocks of the social structure of Fedora. They're very informal with an incredibly low bar to creation. We used to have a complicated process, but no one was really following it, so here's this page instead.
A SIG may develop over time into a more formal Subproject, but that isn't always the case, or even the goal.
To create a SIG:
1. Make sure there's enough interest from other people
Post to one or more relevant Fedora mailing lists — if you're unsure, try the main devel list. Remember that a cardinal rule in any volunteer, community project is that you can't make anyone else do work for you — even if your idea seems like the best thing ever, it won't happen automatically without someone to turn the crank.
A good initial message will lay out the proposed mission or goal for your new SIG, and point to any existing concrete, practical plans. Your message title might be something like: "Software Defined Networking: interested in a Fedora SIG to work on this?" or "Fedora on Mars? A proposal for an InterPlaNet SIG"
2. Create a wiki page
If the response is good, the next step is to create a wiki page for the SIG.
Take a look at some of the existing SIGs listed on https://fedoraproject.org/wiki/Category:SIGs. Create a page like that, and link it in the list. To edit a wiki page, you need a Fedora account, and Help:Editing access. (If you're new and have trouble with this, the Join Fedora SIG can help!)
3. Request any additional resources
See the Request For Resources page, for setting up things like a ticketing system or a mailing list. (But, think twice about whether a new mailing list will be necessary or whether it will serve mostly to split the conversation.)
4. Start things rolling
It's easy for a new SIG to lose momentum. In Fedora, we use [How_to_use_IRC|IRC (Internet Relay Chat)] heavily, and scheduling a weeky or every-other-week meeting is a good way to establish a cadence. You don't have to do everything there (and, with a global community, coordinating in mailing lists is usually better!), but it can help make sure nothing stagnates.
5. Do awesome things
6. But, also...
If your SIG becomes inactive — and that's perfectly normal as interests and priorities change — please remember to mark it as "inactive" on the wiki page you created in step #2. Hopefully, this won't ever be needed, though, so go back to step #5 now!