The idea is to stress the metal up into a bowl shape without having to use any welding or complicated tools. By making a few slits here and holes there, you can bend, fold, and pin the metal together pretty quickly with some tin snips and a drill.
Assuming the sign is free, only a couple bucks' worth of fasteners are needed. Tin snips or a Dremel, pliers, a drill, a wrench, a screwdriver, and a ratchet would be handy to have close. Only takes an hour or two; most of the time is actually laying out the cuts more so than the actual construction.
It uses the same basic principles of this chair on a smaller scale: http://www.instructables.com/id/Nine_Square_Chair/
For sale here: http://www.etsy.com/view_listing.php?listing_id=28562050