It's really quite simple. You take a person's most marked physical feature, compare it to genitalia (male, female or animal), and end with the suffix -bag, -wipe or -muncher. You can also add an optional reference to excretion from any of the glorious openings offered by the human body.
Give a man an insult and he can hurt people for a day.
Teach a man to insult and he can hurt people who tease him because he never learned to fish. - Lem