If that's all you want to do, then as pittrek says, the process is not difficult.

Here's what I'd do:
1. Use DVD Decrypter in IFO mode to demux video (M2V) and audio (AC3) files.
2. Open M2V into DGIndex and save as D2V project.
3. Write AVISynth script in notepad to convert video into 16:9 and add in subtitles where needed.
4. Open AVS file in CCE and encode new video.
5. Author new DVD using new video file and the AC3 file from step 1.

So are you having a problem with step 4? Did you preview the AVS file in VirtualDub or a media player to make sure it was correct? What settings did you use in CCE?