Many expert recommend against making plaster casts of sensitive body parts. However, if you do, SHAVE. AND USE LOTS OF VASELINE. Mike and Zeron explain how to make a mold of one's testicles. http://www.open2.net/healtheducation/body_mind/cast_yourself.html Frankly, this seems like one of the most detailed explanations I've found, so I would advise following this.
A process by which a liquid material is usually poured into a mold,
which contains a hollow cavity of the desired shape, and then allowed
to solidify. The solidified part is also known as a casting, which is
ejected or broken out of the mold to complete the process. Can be used to make truly unique toys.