Measure in 4 inches from the end corner of.
Install gutter downpipe.
Place a step ladder under the area you are installing the downspout.
Fit the gutter in place which normally just clips in with the appropriate stop ends connectors and gutter outlet for the downpipe.
How to add a downspout in an existing gutter step 1.
Gutters over flows add downspout to bring water down from roof faster.
Cut it to length with a hacksaw and removing any burrs with a half round file.
Watch and learn how and stop.