Side vent chimney for gas fired furnace?

Work from the inside out to make sure you exit the house in between studs When the duct is all connected and you're ready to cut the outside use the duct as a template and make a circle on the inside of the sheathing, put a screw through the wall in the center of the circle, from the inside out. This will show you exactly where your going to make the hole outside. Draw the proper diameter circle on the siding and cut the siding with a utility knife first, this will prevent chipping, then cut the sheathing with sawz-all or jigsaw. Make sure you use high heat caulking against the siding and all the duct joints. If it wasn't provided with the kit check with the people you bought it from, don't want the fumes (CO2) leaking in the garage, might go BOOM. Hope I was some help.