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.
Step 1: CAD prep
First, ensure you have a "solid" object, rather than abutting surfaces. The object must be theoretically water-tight. Save it as a new file (I just call mine 'originalname-MOLD.3dm'). Next, consider the orientation of the mold. It doesn't have to be printed such that the object is in its expected upright position, it can be at any angle that is suitable for casting. Do you have a flat bottom, such that you can make a one-piece mold, or do you need a parting line, the join where the mold will come apart to extract the casting? If so, where is the best place for it? In general this should be at the widest part of the object, and it is often easier to extract the casting if the parting line runs more or less along the length, rather than width or height, of the object. If you have undercuts in the object, significant concave areas, consider placing the parting line through or near these. Good parting line placement is a bit of an art.
If the parting line has to be highly shaped, the technique becomes more difficult; for this simple tutorial I am assuming a straight parting line, either in one plane or only on a few intersecting planes. If you are planning cast with a flexible material such as silicone or foam, you can get away with fudging some of these guidelines a small amount as I did in the example.