This tutorial assumes that you already have uploaded the video to your YouTube channel and that you already have a written transcript of the audio from your video. If you have an .srt file that you need to synchronize with your video, see Adding Caption Files to YouTube Videos. If you do not have a transcription file prepared, use the Creating Captions in YouTube from Scratch tutorial instead.
Make sure that your transcript is saved as a .txt file. You can save it as a .txt file in either TextEdit (Mac) or Notepad (Windows).
On the video page click the blue button that says Add new subtitles or CC, and select the language that your subtitles are written in from the drop-down box.
In the window that pops up, click Upload a file.
Select the radio button that says Transcript, click Choose file underneath it, find the .txt transcript file on your computer, then click Upload.
You are now on a page where you can edit the transcript. Click the button that says Set timings. This may take a few minutes.
To edit the content of the captions, click the caption you'd like to edit left of the video and start typing.
You can adjust the start and end times of each caption at the bottom of the screen by clicking & dragging on the edges of the white boxes with text. The audio signal is shown in grey behind the white text boxes to help find where the audio begins and ends. Hit Shift-Enter to add a line break; you should have no more than 2 lines of text in your captions. Try to split up your captions by complete phrases.
When finished, click Publish in the top right corner to finalize the captions.
Now you can go to your video and check to make sure that the white CC button appears in the bottom-right corner of the player, and that your captions appear when you click on it.
If you have captions in multiple languages, you can switch between them by clicking on the Settings gear icon in the bottom right corner of the player, and clicking on Subtitles / CC.
If you need to edit your captions further, you can click on the caption file from the Manage Subtitles and Closed Captions screen. When you get back to the caption editing screen, it will be all greyed out, and you need to unpublish the file before you can edit it.
If you need to export a caption file to add to Kaltura or Vimeo or another media-hosting site, you can download an SRT file from the caption editing screen by opening the Actions drop down menu and under Download selecting .SRT.