Making silicone molds seems easy, but there are a lot of missteps to be made along the way that can mean the difference between a great, reusable mold, and one that’s a sad waste of silicone.

The process looks simple enough, and if you have a 3D printer, you only need some lead-free ... part and make the entire part a hole inside an enveloping shape. Once you print the mold and fill ...