To do this, the first step is to register yourself on the web page (very quick process). Once you've done this, you only have to paste the URL of the video you're interested in in the bar developed for it and click on the option 'Submit'. You will be directed to a window from where you can watch the video... and create notes.
Each time you click on the 'Tag it' button, the video will stop and automatically show the exact minute and second where it stopped in a small text window that allows you to add a short comment. This way, you can take as much notes as you want until the video is finished.
Once you've taken all the notes you only have to save the result. That way, you will obtain a URL that can be passed on to friends with the video perfectly commented by you. Perfect for large clips that only have a few interesting moments.