The first two lines do the job
YouTube shows a short preview of the description before someone clicks more. Those lines should say what the video delivers, in the same words as the title. A description that starts with a list of social links hides the promise. Put links after the summary.
This is the description on a video. The channel description, the text in the header and on About, is a different field. The Data API caps that one at 1,000 characters. The bio generator drafts it, and how to write a YouTube channel description covers the first sentence. The description generator drafts the video opening from the topic you type. Edit it so it matches the video you filmed.
Add chapters on longer videos
If the video is long enough to skip around, add timestamp lines. The first one should be 0:00, then at least two more. YouTube uses that pattern for chapters. A list of times that starts at 1:30 does not. Name each chapter the way you would say it out loud.
One link, then hashtags if you need them
Link one related video or the playlist this video belongs to. A block of ten links is harder to use than one obvious next click. Hashtags, if you use them, should be the three to five that name the topic. Above 60 hashtags, YouTube ignores all of them. YouTube hashtags vs tags separates that field from the tag box. The caption generator counts the description in bytes.
Run the mini-audit to see which recent uploads have a description under 100 characters or no chapters. That is public data, not a style score.
Save the rewrite with the channel
A description you like in a notes app still has to be pasted onto the video. Paid plans keep the written fixes with the channel audit so the next upload starts from the last one. Auto fix, on Unlimited, can apply a title, description, and tags after you preview them. It does not write the description without that preview.
Questions
How long should a YouTube description be?
Long enough that the first two lines state the promise and a longer video can include chapters. Under 100 characters, search has almost no context. There is no magic character count past that.
Where do links go?
After the summary. One link to a related video or playlist is more useful than a list of every profile you have.
Do hashtags in the description help?
A few that match the video are optional. More than 15 can look spammy. Above 60, YouTube ignores every hashtag on that video.