You'll need an idea of the size, the shape and some idea of what wood you might want to use.
Firstly, I started by finding as many pictures as I could. Also, in looking for some pics I ran into a website where there sizes of the instrument, and a bit more info on how others had made them. This is the link to Yoram Sivan's
Also, this instructable by dimdiode
proved to be very useful for a methodology for constructing the didge.
So, after searching around I drew my own design. The template is as above. The Didge was going to be 40 x 25cm. This worked out at about 141 cm long. I decided that about 7 cm would be suitable width.
After this I managed to get hold of some wood. The darker was Walnut from a local joiners and the light was Pine which I picked up at the local DIY store. The Pine was 3.5 cm thick by about 5 cm wide. This allowed the two halves to be made separately and then glued together.