"James's hat" vs "James' hat". I'd favour the former, although Americans tend to the latter.
"Jesus' hat" is always like that. He's special, dontcha know?
If you're talking about the possessive of a plural, it's simpler - put the apostrophe at the end.
"The fans' cheers were loud."
"The girls' bosoms were pleasing to the eye"
There. Hope that clears things up.