Depending on your system, you may be automatically logged in when you go to YouTube
Note: YouTube recognizes the active gmail account when you open it. You may need to sign out and sign in with the correct account if you have multiple accounts.
From the Upload menu, select Video Manager
Click the checkbox beside the video you want to download
From the Edit list, select Download MP4
Note: Some systems have a folder where things are automatically downloaded (although I don’t recommend this because it is too easy to “lose” the files). For techie mystery reasons, it puts this kind of gobbldey-gook in there and that makes it very hard to search for when you need to.
Please log in again. The login page will open in a new tab. After logging in you can close it and return to this page.