To determine whether you need to add -s, follow these steps:

1. Write down the possessor or possessors.  Examples:

2. Put an apostrophe after the possessor or possessors. Examples: 

3. Add the noun that is being possessed after the word,
and pronounce the two words aloud to find out whether you add
an additional "s" sound to the possessor when you say it. If you do,
add an "s" after the apostrophe.

Examples:

As you can see in these examples, the use of an apostrophe is
pronunciation driven and thus individualized, not determined by hard
and fast rules in all cases.