Possessive with "s"

To indicate possession for people or animals :
1. Singular - Add 's :
Example - Susan's pen
- Thomas's house .
2. Plurals - Ending with s' :
Example - The students' mother
- The ladies' hats .
3. Other Plurals - Add 's :
Example - The children's bags
- The men's cars.

Reference: Grammar Practice for Pre-Intermediate Students.