Hold the gutter sleeve against the gutter in the desired location of the downspout and trace an outline of the sleeve onto the gutter using a permanent marker. Next, use metal sheers to cut along the outlined sleeve tracing creating a gap in the gutter where the sleeve will rest. Apply a ribbon of sealant along the outer rim of the sleeve and insert the sleeve into the cut-out notch in the gutter. Secure the sleeve to the gutter using a metal screw on both of the top corners of the sleeve. Allow the sealant adequate time to set before proceeding to the next step.
Attach an elbow to the sleeve with a screw on each side. The angle of the elbow should point away from the gutter and toward the house. Next, secure an elbow extension fitting with screws into the first elbow and align it perpendicularly to the house. Install the second elbow by attaching it to the bottom of the elbow extension. The second elbow should be as flush to the house as possible.
Insert the downspout into the second elbow and secure with screws. The downspout should extend from the second elbow to about six inches above the ground. If necessary, use the hacksaw to cut the downspout to the proper length. Next, install an elbow at the bottom of the downspout to carry the water to the ground.
Wrap a metal strap around the downspout just below the connection of the downspout and second elbow. Screw each side of the metal strap to the house. Repeat this step to install several metal straps along the downspout. The placement of the metal straps can vary between 5 feet and 8 feet apart, but no more than 10 feet apart.
Check the downspout for leaks by using a water hose and flushing the gutter just beside the downspout. If you discover any leaks, patch them by applying a small amount of plastic cement. Apply the cement in a thin layer and allow it to fully dry before testing again for leaks.