Now that we have the rules out of the way, let's cover some exceptions. Ancient Greek names, such as Hippocrates, Socrates, and Euripides, use only the apostrophe. While it was Charles's book, it was Socrates' pen that wrote it. The biblical Jesus and Moses are also exceptions, though I have heard it suggested that, instead of writing Moses' laws, it would be better to write the laws of Moses. A stylistic preference, not a law.
Simple, right? Add 's to make a singular noun plural.