You can use the bookmarks feature to mark an interesting event in a gameplay video. You can also use it to stitch together a highlight reel, kind of like a montage of a memorable game session. Here's how you do it:

Before you start playing your game, set up your preferences to decide if you want to record with Automatic recording or use the Manual recording feature using a hotkey. 

You should probably check your Bookmark hotkey just to see what the hotkey is.

Go ahead and start your game session. When something interesting happens, hit the hotkey to bookmark that moment. The default hotkey is Ctrl + F2. You'll see a bookmark notification in the upper-left corner of the screen.

When your game session is complete, you can go into your recorded Game Session videos, and you will see your video now has the bookmark icons along the match timeline. Hovering over the bookmark will show you the timestamp for that moment. Clicking on them will activate that duration of the timeline below.

You can also use the 'Add Bookmark' button that is on the red dot for the current frame. Hitting that button will create a new bookmark to use in a highlight reel.

To stitch together a highlight reel, follow these steps:

Select the first bookmark pin that you want to be in the highlight reel. Set that clip's duration by dragging the edges of the yellow frame. 

CTRL-click on the next bookmark. Set its duration with the yellow frame as well. (You must set the duration before you move on to the next bookmark.)

Don't worry about accidentally choosing uninteresting bookmarks. If you have chosen a wrong bookmark, CTRL- Clicking on a bookmark again will deselect it from the chosen bookmarks.

Keep going with the selecting of bookmarks and setting their duration. When you are done selecting bookmarks, you can preview your highlight reel by choosing the lower play button.

When you are done, you can hit the button called 'Save to' to upload it to the website for sharing with others.