The gutter installation finishes with the installation of a half-round gutter end cap. You install an end cap at the end where the gutter doesn’t turn a corner. In the end you add the downspout a few inches inside the end cap.
First you need to take the end cap and mount it on the end of the gutter. Then drill a sheet metal screw into the cap to fix it in place. After you do this, you must add a pop rivet with the use of a pop rivet gun. The pop rivet goes inside a 1/8 inch hole that you must do with a drill. You can buy your pop rivet gun from any home improvement store in any size you want. The best flush and watertight fastener you can out inside the gutter is the pop rivet. In the end take off the screw and repeat the steps to add a pop rivet.
The end cap should be completely watertight and should not allow any water leaks since you will add a downspout to the gutter. To completely seal the end cap and rivets you can sue silicone caulk.