Measure the length of the vertical section of the stovepipe in which you will install the damper.
Mark the spot roughly halfway up the section.
Measure the circumference of the pipe section, starting from the spot you marked and ending back at the spot.
Divide this number by two, and measure from the starting point to this figure, which should be on the other side of the pipe directly opposite the first mark, and mark the second spot.
Drill a hole through the pipe on each side where you made the marks, moving the drill in and out a few times to smooth the edges.
Take the stove pipe apart, remove the section with the holes, and set it on newspaper to catch any soot and other debris.
Twist the center rod of the damper to separate the two pieces.
Insert the tip of the rod into the first hole you drilled, and lower the damper into the stove pipe.
Insert the rod through the damper all the way to the end, push the tip through the second hole in the stove pipe, and twist the rod to lock it into place so that both the damper and the rod handle are in a horizontal position.
Turn the damper handle a few times to make sure that it moves freely inside the pipe.
Reconnect the pipe section that contains the damper to the other sections, and screw it into place.