From inside the attic, select a spot on the sidewall for the flexible duct to exit the house. The location should be between two wall studs and within 6 feet of the vent fan.
Take a couple of reference measurements so that you can locate the spot from outside—from a nearby window, soffit, or other feature you can access from outdoors.
Mark the hole location on the siding, using the reference measurements.
Cut the duct hole through the house wall using a 4-inch hole saw.