What a working chapter list looks like
Paste the list anywhere in the description, one chapter per line, each starting with its time:
0:00 Intro
0:45 What you need
2:10 Step 1: Cut the boards
6:30 Step 2: Assemble the frame
11:05 Finishing and paint
14:20 Final resultText before the list is fine, and a dash or colon after the time is optional. What breaks chapters is a first line that is not 0:00, a chapter shorter than 10 seconds, times out of order, or fewer than three entries.
Linking to an exact moment in a video
A timestamp link opens a video at a set point. Add ?t= and the time in seconds to a short link, or &t= to a full watch link:
https://youtu.be/VIDEO_ID?t=135
https://www.youtube.com/watch?v=VIDEO_ID&t=2m15sBoth start at 2:15. On a computer you can also right-click the playing video and choose “Copy video URL at current time”, or tick “Start at” in the Share box. On the phone app, the Share button has no start time option, so add ?t= by hand.
Timestamps in comments
Any time written in a comment, such as 4:32 or 1:02:15, turns into a link that jumps to that moment. Viewers use it to point to their favourite part, and creators can pin a comment with chapters for a video that already went live without them. Comment timestamps do not create chapters on the progress bar, though; only the description does that.
Adding chapters from your phone
Open the YouTube Studio app, pick the video, tap the pencil to edit, and paste the chapter list into the description. Save, and the chapters appear on the progress bar within a few minutes. You can build and check the list here on the phone first, then copy it across.
Automatic chapters and key moments
YouTube can also make chapters by itself for videos without a list. They can be useful, but the titles are guessed and the split points are not always where you would put them. Chapters you write yourself always replace the automatic ones. If you never want automatic chapters, turn off “Allow automatic chapters” in the video’s details in YouTube Studio.
Google search can show chapters as “key moments” under a video result, which gives a long video more ways to be found for specific questions. Short, specific chapter titles such as “Replacing the filter” work better there than “Part 2”.