Use commas to offset interrupters.

An interrupter is a word or group of words that tells more about (modifies) something in the sentence but is not essential to the meaning of the sentence.

Grammar Bytes defines an interrupter as a word or group of words that "significantly breaks the flow of a sentence."

Any football player not practicing will be kicked off the squad.
This sentence requires no commas because the phrase "not practicing" is essential to the meaning of the sentence.  "Any football player will be kicked off the squad" means something different.

John Reynolds, who was not practicing, was kicked off the squad.
This sentence requires commas because "who was not practicing" could be removed from the sentence and the main idea would still be the same: John Reynolds was kicked off the squad.

I want you to clean your room, Thomas, before you leave the house.
Thomas could be removed, but the sentence would retain its meaning; therefore commas are needed.

The young lady named Brooke, if you must know, is very sweet.
The words "if you must know" can be removed without changing the essential meaning of the sentence.

That movie, which I was forced to sit through twice because my sister wouldn't give me a ride home, was awful.
You could just say, "That movie was awful."  The essential meaning is the same.

Move on to Rule 4.