Learn to mount end caps to steel gutters
The gutter functionality and also the aesthetics of the gutter depend on whether you have end caps installed on the gutter or not. The gutter would have no reason to be installed if it didn’t have end caps installed because all the rain water would just fall through that hole left uncovered. Below is a simple description of the end cap installation process.
The end caps are installed on those corners that do not make a turn and combine with another gutter part. You can either use galvanized screws or pop rivets to attach the end caps to the gutter end. If you choose the screws, all you need to do is drill them inside each end cap attaching them to the gutter this way.
Still the rivets are a better choice than screws because they perform better and look better also. To install them pop the rivets in the holes while holding the end cap with one screw that you will replace at the end with a rivet.
Use caulk to seal the part where the end cap and the gutter connect. If you really want a tight seal you shouldn’t skip this step.