A video of the cardboard uke in action can be found on YouTube.
Step 1: Materials...
Several large shipping boxes made from corrugated cardboard. The tighter the corrugation the better.
Several sheets of non-corrugated composite cardboard like cereal boxes or the back of notepads
2 fully threaded bolts or a threaded rod
4 geared guitar or ukulele tuners with flanged bushings
1 set of soprano ukulele strings
12 1.5" long cotter pins
2 small hinges
1 bottle of wood glue
2 wood screws (1.5 - 2 inches)
Sharp hobby blade