In this Instructable I'll share what I learned about how to draw a CAD for a mold and 3D printing it out. I will assume you have access to and moderate proficiency with a CAD system (it doesn't matter what kind). For the printing, if you want to make your own but don't have access to a 3D printer, there are many services which print for a fee, or Instructables may be able to do that for you. The actual casting I will leave out as I was using an expanding polyurethane foam, which is not a generally applicable material.
Unfortunately my print picture set and my mold CAD picture set do not show the same mold. The pictures of the print, in the second half of the instructable, are of a slightly more complex mold, with a two-plane parting line. The process I outline is the same for both, however.