I think the main idea for you is to rip your DVDs to VOB or Mpeg2 format with OSEx
so that you don't degrade the video quality. VOB / Mpeg2 is the native DVD video format. Encoding to avi will reduce the quality. From there you will need to join and or split (edit) the VOB or Mpeg2 files per episode. I recommend using MPEG Streamclip
for that task. Once that is done, you will need to play the edited video file with its corresponding subtitle file in a program like VLC
to check if the new subtitles you want to use are in sync. If the subtitles are not in sync then you will need to use Jubler
to adjust the timing offset of the subtitles to match the video. BTW, Jubler is a Java application, so you will need to make sure you have the latest Java Virtual Machine installed. If you have been keeping up with Mac OS X updates, then you should be up to date, so no worries. If you have not been updating Mac OS X, then click the Apple menu (top left corner of desktop) and select "Software Update...". Be sure to know your administrator password. You may need to perform the software update several times until you get a message window indicating there are no further updates or that your software is up to date (can't remember which). Ok, you've ripped, edited, updated your Mac, and checked and/or re-synced the subtitles. So far so good, or you've pulled all your hair out because this is some crazy project you want to get done and there is a bit of a learning curve... Well the fun is not over yet. You still need to author your new DVD with Sizzle
I guess that covers it, all the software I mentioned is freeware, some of it is old (vintage 2004 - old? In todays digital age, software just 2 years old might be considered ancient).
Anyway hope this helps. I pointed out all the tools and a logical workflow. Once you get some working knowledge, please document your steps and share your knowledge with others here.