How do you write Williams in possessive form?

There are two accepted forms for possessive singular nouns that end with an s:

Add an apostrophe (') after the existing s at the end of the Word: Williams'

Add an apostrophe s ('s) after the existing s at the end of the Word: Williams's

Examples:

Jim Williams' house is number four.

Jim Williams's house is number four.

The possessive form of the singular noun William is William's.

Example: I like William's new car.