To cover the ends of the gutter that don’t connect with anything you need to install end caps. The end cap is installed only at the highest point of the gutter so that the water drains properly. You need to fit the cap exactly with the gutter.
First, before starting to fasten the pieces, you need to join them together. Place the back end of the cap over the back end of the gutter. It is very hard to push the bottom of the cap on because the pieces have irregular shapes and the joint is very narrow. So you will need a lot of patience and mastery to perform this job. After putting the cap into position, you need to use the crimping tool or the rivet gun to fasten the cap into place.
In the end you need to seal the seam that remains between the end cap and the gutter. For this you can use a caulking gun and silicone sealant.