Learn to cut a downspout
The purpose of the downspout is to carry all the water that comes through the gutter around the house away from it and into a basin or in the ground. Usually people install downspouts along house corners because this way they blend better with the house. After you decide to install a downspout you will also need to cut it so that it fits in.
Measuring the length of the downspout you need can be done using a tape measure but it would be a lot easier to just put the downspout in place and mark the edges where you need to cut it with a pencil. This way is also more accurate.
When you are finished measuring you will have four lines on each side of the gutter. Transfer the lines around the gutter using a square.
Place the rain gutter downspout on the miter saw table and line up the marker line with the saw blade. Now cut the vinyl carefully using the saw.