When it comes to possessives — I usually have to think about where to put the apostrophe.

Here’s the rule:

Form the possessive singular of nouns by adding     ‘s

Marge’s cool bicycle is clean and spiffy.

The cat’s meow was annoying.

Of course there are exceptions ––
When the possessives of ancient proper names ends in  -es and -is,

• Jesus’
• for conscience’ sake
• for righteousness’ sake

But such forms as Moses’ Laws, Isis’ temple are commonly replaced by

• the laws of Moses
• the temple of Isis

This information was found in The Elements of Style – (Strunk & White)