Remove all debris or chimney cap from the existing chimney, exposing the top part of the chimney.
Place the chimney flange over the chimney opening, and mark the location of the fastening holes. Check to ensure the chimney flange opening is not smaller than the chimney opening.
Drill holes in the chimney masonry at the marks made from the chimney flange using an electric drill and masonry bit.
Place the chimney flange over the drilled holes, and secure the flange to the top of the chimney using bolts (usually provided with the flange).
Add the flue pipe extender to the chimney flange, and secure it with the side screws (usually provided with the extender).