If you mean rip them and use the exact same subtitle files on a fanmade DVD, and not OCR'd SubRip files, it's actually pretty complicated but it gets easier after you've done it a couple times. I've done it for a couple anime DVD's using someone elses guide, it can be found here: http://members.cox.net/~sensuifu/subyua.html (all programs used are freeware)

Discussion and some additional tips (including some improvements making the timings more precise) here: http://www.animeondvd.com/forum/showtopic.php?tid/17072/

Incidentally I've only done this for NTSC DVDs, I don't think it's possible to use the exact subs from a PAL DVD on an NTSC one and vice versa.

Let me know if you need any help or if you have tips on improving the process.

Oh, and if you just need help in using SubRip, the above guide shows how to use it a little as well (just ignore the part where it says to type anything during the OCR process).