What file format should I use?
Unless otherwise specified, you want a .srt file. They MUST follow this format:
00:00:15,960 --> 00:00:18,820
00:00:18,821 --> 00:00:20,900
It will fail if you use any spacing other than:
Client wants to see two languages at the same time?
Use this tool to merge two identically time-coded SRT files into one.
How do I 'apply' the subtitles?
If the video is on YouTube, simply choose the "CC" option and add subtitles. Note that this requires the video owner to have opted-in to "Allow community contributions". If this hasn't been allowed, simply email the .srt file to email@example.com and we will handle the rest.
If the video is a locally saved file, we recommend using the open-source software Handbreak and the "Burned In" option to affix the subtitles. Note that any subtitles with non-alphabetic characters (like Chinese) will require you to toggle the encoding option to UTC-8.
What if I just have a video and need to create subtitles and/or timecodes?
Here's a hack I wrote about to use YouTube to do this almost instantly.