Assemble the vent pipe and dry-fit it on the exhaust port on top of the tankless heating unit. Mark the spot on the wall where the center of the vent pipe comes into contact with the wall.
Remove the vent pipe from the top of the heating unit.
Drill a small pilot hole through the wall to the outside.
Go outside to the pilot hole. Hold the metal wall thimble against the exterior wall and trace the outline of the thimble on the wall.
Cut an opening in the wall for the vent. Start by drilling a 1-inch hole with a spade bit near the top of the circle you traced. Cut the rest of the circle with a reciprocating saw, starting in the 1-inch hole.
Hold the flange thimble against the wall. Remove any strips of siding that prevent the thimble from resting flush against the wall.
Apply a thick bead of silicone caulk around the edge of the flange, then slide it into the hole in the wall. Screw the flange to the wall through the holes around the perimeter of the flange, and cover the screws with the silicone caulk.
Go inside and connect the flange to the vent pipe.
Re-install any strips of siding that you had to remove to install the flange.