How do you use boys' in a sentence?

Used in that way it refers to a possessive of more than one boy. So if there were a lot of boys playing in the mud and all of their shoes got dirty, you could have the following sentences:

The boys' shoes were very dirty. The boys' shoes are as dirty as a muddy puddle.

If it was just one boy whose shoes got dirty it would be:

The boy's shoes were very dirty. The boy's shoes are as dirty as a muddy puddle.