Before anything, you need to download these 2 free softwares:
They don't need instalation.
Run SubToSup. Click on [....] or File and then Open.
Search folder where is your IDx file, and click twice for opening.
It will appear SubToSup 0.9 Beta window, just click OK and close SubToSup.
It will appear subtitle with different name in folder.. (example: subtitle-id_en_0x20.sup)
Now run DVDSubEdit. Drag your subtitle-id_en_0x20.sup file in DVDSubEdit window (or in File-Open .SUP file)
It will open another window requesting IFO file, just click cancel and continue.
Lock now in right bottom corner of DVDSubEdit window, there is a button "Run OCR".
Clicking on it DVDToSUp is processing your SUP file.
When it's done, you can click on "Save as .srt", and "Save OCR output as .srt file".
Note: this is only for subtitles on english languague and in idx/sub format. It works 100%
After all that usually subs will be little out of sync. They can be adjusted in lots of programs, i will explain it with Subtitle Workshop 2.51
Load your subtitle with CTRL+O and movie with CTRL+P.
You will notice small buttons 1 and 2.
Find some dialog in the begining of the video, make that line blue, wait that sentence to be spoken, and click on button "1", like in the image
After that go to sommewhere to the end of the movie, repeat procedure, just this time click on button "2"
Note: If your subtitle is from same release like video file, that should work 100%, or if you're trying to syncronize to another DVDRip release it works in 97% of the case. Usualy syncronization from TV, VHS, HDTV -> DVD or Blu Ray is not working (Sometimes it is).