Marsie/Martlen - a good idea is to try out the FM that's available to non-subscribers. It gives you an idea of how these are different from the regular cases.
Another thing you can do is look at the form Ben has posted under "Submitting Content" - this will give you an idea of what it is that's needed, which parts and things like that.
Also John Hale (Michael Bagen, the guy who's written most of the brilliant FMs that you see here on Sleuth) gave this advice to someone the day before yesterday: "The trick with a feature mystery is to lay out the basic points of the story (beginning, twist, and end). From there, you figure out the mechanics of the mystery (what's revealed and now, and what that unlocks). It's basically a logic chain fused with a story."
Hope you can use some of it :)