Once your DVD has been inserted, navigate to it and select it (select the DVD itself, not its tracks or contents).
Handbrake will detect it and scan its contents. Once this is done, you'll be able to choose whether to rip part of the DVD or all of its contents. If you're converting a feature film, ripping the entire DVD probably makes sense, whereas with a TV show, you may just want a few episodes.
Handbrake also allows you to rip alternate audio and video tracks, such as subtitles.
