A word of caution: Halfway through the cutting process, I found myself openly objectifying cardboard. "Oh, that's a beautiful piece..." "That crease simply won't do." I also experienced cramping of the hand muscles. If you choose to hand-cut the cardboard, I STRONGLY recommend selecting a project with few pieces and no fine details.
Step 1: Find a suitable .stl or .obj model
This Sphinx of Hatshepsut was modeled by beekeeper on Thingiverse. Interestingly, this model was created using another 123D product, 123D Catch. The modeler took somewhere between thirty and seventy photos of the 15,000-pound granite sphinx at the Metropolitan Museum of Art in New York and used the software to stitch the photos together into a three-dimensional model.
123D Make will accept any .stl or .obj file (found all over model repositories such as Thingiverse and GrabCAD), which describe the surface geometry of a three-dimensional model.