Upload your video to YouTube and access the Video Manager screen. Next to your video is a drop down - select 'Subtitles and CC'.
You may be required to select a video language. Select the preferred language and 'Set Language'
Select the option to add new subtitles or CC and choose an appropriate language.
There are three methods available to add subtitles or closed captions:
Upload a text transcript or timed subtitles file. YouTube has clear instructions on creating subtitle files to upload.
This method requires you to have a full copy of your time-coded transcript to upload.
Type or paste in a full transcript of the video and subtitle timings will be set automatically. The video will be displayed to the side of the screen with an empty text area for you to type your caption dialogue to the other.
The 'Actions' dialog lets you delete the subtitles, download the file for off-site editing or storage and to upload a caption file.
Create subtitles and closed captions by typing them in as you watch the video. This option will display the text as a caption over the video and in the 'Transcribe & Sync' editor underneath as you type. Adjustments to text and timing can be made as the video progresses.
This option is more suited to users with some experience in creating captions as it progresses quite quickly.