Truy cập trang web YouTube. Mở video mong muốn. Nhấp vào video để tạm dừng phát rồi nhìn vào thời gian. Cuộn xuống và nhấp vào "Add a public comment". Gõ mốc thời gian hiện tại xuất hiện trong video. Nhấp Comment.

I'm interested in that, but I only have a few mins. Can you give me at most the first 500 characters of a detailed explanation in English about that?
YouTube will open to the Home page if you're already signed in. If you aren't already signed into YouTube, click Sign in in the top-right corner of the page, enter your email address and password, and click Sign in again. You can do so by typing the video's name into the search bar near the top of the page and pressing ↵ Enter (or ⏎ Return), or you can select a video from the Home page. You'll see the time stamp near the bottom-left corner of the video window, just right of the volume icon; it w