Step 1: Remove Downspout from Boot
Unless your gutter downspout can be easily lifted out of the boot, you’ll need to cut it just a few inches above the boot—making sure that you’ve left enough room for the downspout “elbow” or other end-of-pipe device.
Step 2: Seal the Boot
The boot must be permanently sealed to keep water from entering the sewer line. There are different ways to seal the pipe so before proceeding, check to find out what approach is used in your area. Some jurisdictions suggest the following method:
"The pipe can be sealed with concrete or other material that makes a permanent, weatherproof seal. Before starting, check the pipe to make sure that you will not block any other junction (such as your washing machine draining into the same pipe). The push balled-up newspaper tightly three or four inches down into the boot. Make sure that the fit is snug, otherwise the concrete could actually block the sewer. Carefully pour the concrete or other material that gives a permanent weatherproof seal into the pipe, and smooth the top surface."
Other areas do not recommend using newspaper and suggest the use of friction or sewer plugs, which are often available from licensed contractors.
Step 3: Redirect Rainwater With End-of-Pipe Device(s)
The last step is the redirect the rainwater away from the house. The exact approach depends on individual conditions. It is important that the rainwater discharges your foundation (three or four feet is usually enough), that the run-off is not a nuisance to your neighbors, or causes other problems, like discharging across a sidewalk so that ice forms in the winter.
Sometimes all that is needed is an “elbow” pipe to the bottom of the gutter downspout and a plastic or concrete splash block. Other situations (such as sloping ground) may require a different solution. There are a number of devices available designed to channel the rainwater away from your home (ask a hardware store to recommend the best one for your needs).
Check with your local sewer system authority or public works department for other redirection options in your area.