How to create a video part from uploaded video
On top right of the video there will be three dot ···, click it, then click on Post video part
Note that this option is only for owner or friend of owner. The owner must add you as a friend.
You can change manually Start or End time to fit precisely. You can use loop button to preview the video part.
Once you have fit the video part enter a proper title and description. Press Save and Publish, done.