For great instructions on how to tie many different globe knots using a mandrel I recommend The Globe Knot Cookbook, by Don Burrhus: http://knottool.com/gk_kit.html
Step 1: Materials
I recommend getting it in the white, strong and flexible material as we'll be sticking pins in it. When I received mine, there was a lot of white powder still in the middle, so I recommend rinsing it in water to get it out. Then let it dry before you begin tying.
Or, if you have a 3D printer you can print the attached stl file.
You'll also want some T-pins and some kind of small diameter cord. I'll be using lacrosse crosslace which can be ordered online in many different colors: http://bit.ly/15vKcXg