2. Click the video description to expand it
3. Scroll down and click the tiny "Show Transcript" button near the bottom (whoever decided to bury it down here was very misguided)
4. Ctrl-F and search any word. Occurrences in the transcript will be highlighted and you can press enter to scroll the transcript to the next one. Click the transcript to seek the video.
I see that this extension shows occurrences on the seek bar which is cool. There is also a slight problem with regular ctrl-F: if you search for a multiple word phrase you might not find it if the phrase happened to be split between two chunks of the transcript. So that could be better in this extension. And of course not every YouTube video has captions, but most do these days.