The basic principle is pretty strait forwards. I've booked out my village hall for an evening and let all the local musicians know I'm having a Christmas Jam where everyone can get together for the evening and hack out a few Classic Christmas numbers (Something that really needs doing given the total lack of Christmas cheese predicted for this December's music charts). No Idea how it will go but there's certainly a few people interested.
Just thought I'd mention it in case anyone else wants to try it where they live. If you do let us know how it goes and What Worked and What didn't.