I learned how to make Unusual effects specifically for this jam - this was my first attempt. It's not the best, but I started work on this very late on the jam due to my working hours.
I decided, because I'm in Australia, and the "72hr Summer Jam" takes place in winter here; I'd make an unusual effect themed around that.
(There's a readme in the download anyway.)
Unzip the zip file, and extract the VPK file within. Take that file and paste it into your tf/custom directory.
Upon booting the game, load up the itemtest map (type 'map itemtest' without quotes in console) and spawn in a bot with an unusual hat with the burning flames effect (do this with the 'itemtest' command).