Steps:

  1. Mark the location of a start collar on both the return- and supply-side duct.
  2. Cut holes in each duct using a sheet metal duct cutter attached to a drill/driver.
  3. Screw a start collar to the return-side and supply-side ducts.
  4. Install elbows to each end of a bypass damper.
  5. Attach the bypass damper between the two start collars, effectively connecting the return duct to the supply duct.

Shopping list:

Bypass damper, used to quiet a noisy duct Start collars, for attaching bypass damper to existing duct

Tools:

How to Install a Forced Air Bypass Damper - 82How to Install a Forced Air Bypass Damper - 6