To use the clip editor to share a clip, first you need look in 'Your Videos' and decide which one you want to share. This list is sorted by Game, by time, and the top-most ones are the most recently recorded videos.

With your 'My Session Video' selected, the playback window comes up with a pair of timelines.  The upper timeline is an overview of the entire duration of the Session. The lower timeline is a 'Zoomed Timeline'.

You can click anywhere in the upper timeline to find a moment that you may want to turn into a clip. When you find a spot you like, hit the "Create a Clip" button. A bookmark will be placed in the timeline and a 30 second clip will be shown on the lower Zoom Range and its yellow borders can be moved around.

Clicking on the lower Zoom Range timeline will update the playback window. The white bar represents the current frame. Dragging the white bar around will let you find the action that you want to highlight as a clip.

The left and right edges of the yellow frame can be dragged around to define the start and end point of the desired clip.  The duration of the clip is shown in the upper right corner of the frame.

So now you have at least one bookmark. The bookmark is a saved section of the video. You can keep adding as many bookmarks as you wish.

If you Ctrl-click on each bookmark that you want to include in your video, they will all be included in your saved video. This is how you make montages or highlight reels.

Finally, hitting the 'Save to' button will begin the uploading process for the clip in order to share it. Put a title for the video, as well as a series of hashtags and mentions if you wish.

NOTE: Currently, the title, description and hashtags can't be edited after uploading, so try to get the description the way you want it the first time.