Aegisub

Aegisub is a free, cross-platform open source tool for creating and modifying subtitles. Aegisub features a built-in real-time video preview of the subtitles, along with many powerful tools for timing them to audio and manipulating their appearance.

http://www.aegisub.org/

Ubuntu installation

• Install Aegisub on Ubuntu 12.04

Use

• First open the video, then "Audio->Open Audio from Video", then click play on the video.

Timing

• Introduction: https://www.youtube.com/watch?v=rSiM1Dr9BnM (a bit slower, if you find better one, please share)

Save to Youtube

• Save as file with extension *.srt and upload it to your video.