You need to court a squad that participates in FSO and see if they have a seat you can fill for up to a Frame Set. According to
the rules you can participate in one frame set, three frames to a set, to try out FSO, but after that you would need to join a squad, or, form your own (There may be some minimum numbers for an FSO Squad as the smallest numbers are 4-6 +- 2).
Just remember, it is a "One Life Event" meaning, if you get shot down you’re done for the event. It promotes some very conservative flying practices, and really heightens your sense of nerves. I've come off an FSO jittering from the adrenalin. Other than that It's REALLY fun with MANY aspects to get involved in depending on your level of interest. From just flying a mission, to planning missions, all the way up to being a CIC frame planner.
So, Ask around, I'm sure any squad worth their salt would be glad to have you sit in a frame. They’ll explain what I've said here, and probably a WHOLE bunch more. Just don't show up at 10:45 on Friday night asking if "Anyone has a seat”. Its very bad form, and you probably won’t like the response you get, and I don’t want to see that happen.