First, locate and open a text editor on your computer. If you are using Windows, you can use Notepad or Textpad. If you are using Linux, you can use gedit.
Watch the movie, and jot down the translation as you would like it. Also, you will have to note down the time that you would like the subtitles to appear.
Now put the subtitles in this format:
1
00:04:08.759 --> 00:04:11.595
Morning, Scott.
Morning, Wells.
The "1" means it is the first subtitle. The second line signifies the minutes, seconds and milliseconds that the subtitle should appear on the screen.
Continue on, line for line. For example, in "Top Gun", the next line would be:
2
00:04:15.766 --> 00:04:20.562
Ghost Rider, this is Strike. We have
unknown aircraft, inbound Mustang.
Continue to do so until you complete the film and then save the file with the extension ".srt" instead of ".txt".
When you finish, you may want to upload your file to opensubtitles.org so that other users can find it and benefit from your work.
Note that if the movie you are subtitling is already listed on opensubtitles.org in another language, you can just copy it and reuse the times so that you don't have to go through all the effort of noting the times. You just have to change the language.