Drill a hole in the bottom of the gutter at the 4 inch mark with a 1 inch hole saw attachment to the drill.
Install gutter downpipe.
You can double check the fall with your spirit level.
Gutters over flows add downspout to bring water down from roof faster.
Watch and learn how and stop.
Cut it to length with a hacksaw and removing any burrs with a half round file.
Its easy less than 30 minutes and under 30 00 in parts.
How to add a downspout in an existing gutter step 1.