How do you make a capitalized word a possessive noun?

Proper nouns (names) form their possessive that same as common nouns do.

A singular noun forms the possessive by adding and apostrophe s ('s) to the end of the Word: Carol's
Example: I borrowed Carol's car.

There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the Word: Jess'
Add an apostrophe s ('s) after the existing s at the end of the Word: Jess's

Examples:
I borrowed Jess' car.
I borrowed Jess's car.

A plural noun that ends with an s forms the possessive by adding an apostrophe (') only to the end of the Word: Kennedys'

Example: I borrowed the Kennedys'limo.