To make the subs, you need to use a subtitle creation/timing package such as Sub Station Alpha. If you want to just watch the DVD on your computer with the subtitles overlaid, you need DVDSubber. If you want to remake the DVD so it has the subtitles selectable and displayable on any DVD player, you need to rip the video, probably re-encode it to fit on a single-layer recordable DVD (trust me, double-layer recordables are not worth the effort, expense and lower compatibility), then add on the subtitles using one of the aforementioned authoring packages. But it does beg the question, if you're capable of making your own subtitles for a Japanese-only DVD, surely your grasp of Japanese is sufficient for you to watch it raw anyway?