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)