You only need to click on CC (stands for ‘closed captions’ or subtitles) and you will be presented with three options to:
(1) Upload a transcript file
(2) Paste a full transcript and YouTube will generate subtitle timings
(3) Type as you watch the video
Typing as you watch the video is ill advised, but it is also not as painful as it sounds, because YouTube gives you the option to pause the video whilst you type.
The video below will show you how to create, sync and upload your own video transcript.