Possessive nouns are nouns that show ownership. To make most proper and common nouns possessive nouns we need only add an apostrophe and then an “s” like this:
Sam’s dog has fleas.
Sam’s dog’s fleas are brown.

Possessives ending in s still require an apostrophe and another “s.” Even if a singular noun ends in “s” such as in names like “Gus” or common nouns like “class.”
The class’s field trip was canceled.
Mrs. Glass’s house is near Gus’s office.

Plurals and possessives are a little different. Plural words that are also possessive are usually formed by just adding an apostrophe like this.
The dogs’ leashes are kept in the drawer.
The girls’ basketball team won the game.

If a noun is simply plural and doesn’t show ownership it does not require an apostrophe. For example, none of these words as used in the sentences below require an apostrophe because they are only plural not possessive.
The dogs will go for a walk.
The girls play basketball.

Making an irregular plural noun possessive when it does not end in “s” is very simple. Add an apostrophe then the “s” just as if it were a singular noun like this.
The children’s book is lost.
The fox’s den is in the woods.

Various situations in possessives grammar call for a few seemingly tricky rules. Showing possession when two nouns are joined together and possession is shared is different from indicating possession when two nouns are joined together and ownership is separate. When two people are listed as owning one thing together the possession is shown on the second noun only. Add an apostrophe “s” to the second noun, and leave the first one as it is. For example:
John and Susan’s house is a fun place to visit.
Sally and Tom’s game was left on the floor.

If the two nouns have separate ownership of two separate objects, each noun needs an apostrophe and an “s” added to them. For Example:
Susan’s and Tom’s dinner plates are still on the table.
The dog’s and cat’s water bowls are on the floor.