I always use Subtitle Workshop. You can edit DVD subtitles easy (you need to rip them first and convert them to a readable format, but there are many sites that you can get sub tracks ready to use) and you can create your own too. It has a preview window so you can check to see if everything is syncing up. but as long as you have the sub track that matches the framerate of the video then you should have no sync problems. You can also convert the finished subs to both PAL & NTSC timecoded tracks at the click of a button.