A little flick of the hand in the direction you would naturally bat, while making a "shhhiou" sound with your mouth. Much like when someone does it over their head.
You say "Swing n a Miss" directly after the swing - all in 1 word.
Jim: I think you mean "bored" broad.
Brett: Yeah, because she doesn't have any food!
Ryan & Brett: *shhhiou* Swing n a Miss!