How do you use an apostrophe to show possession?
Apostrophe-s for singular, s-apostrophe for plural.
If something belongs to Bill Smith, it is Bill Smith's. If it belongs to his family and not just him, it is the Smiths'.
The rule gets mildly complicated if the name in question ends with an s. If something belongs to Jane Jones, it is Jane Jones's, but if it belongs to her family, it is the Joneses'.