Laying a girl on her side, and rubbing your lips back and forth across her vagina and anus while blowing simultaneously, treating her two love holes as you would the holes of a harmonica. To better create the rusty harmonica effect you may also cup her vagina lips and anus in your hand as you would hold a harmonica.
"Dude, I gave your mom the most righteous rusty harmonica last night!"
"Oh yea? What song did you play?"
"Blowin' in the Wind."