This guide was frustratingly confusing, so I'm posting what worked for myself:

[Ayaya28090104]

  This guide was frustratingly confusing, so I'm posting what worked for myself:

1. Download this iOS Shortcut -> uYou Login V2
2. Make sure you're logged out of your Google account on the iPhone (might be optional, but didn't work for me otherwise)
3. Download YouTube from AppStore
4. Open the normal AppStore YouTube App first and go to the login page, afterwards do the same with uYou App
5. Enter email address on both AppStore and uYou, proceed to the password page (do not sign in)
6. Copy uYou YouTube login URL from the title
7. Run uYou Login shortcut and paste URL when prompted
8. VERY IMPORTANT, READ FIRST: You need to go to the original YouTube App and copy the URL very fast, go back to the home screen and uninstall the normal AppStore YouTube app before the prompt appears
9. When prompted, paste normal AppStore YouTube URL
10. A new browser window opens now, login there
11. You'll be asked to open in the YouTube App now, do that (it redirects to uYou)
12. You should already be logged in now, choose your account in the uYou menu if you have multiple
13. Didn't happen to me, but I'll just leave it here: if nothing happens after clicking "open", swipe up/home button

That's what worked for me and makes it much clearer in my opinion. Also, delete that "new video guide", it adds more confusion than it solves because it's heavily outdated. (Logging in with mail and uninstalling YT App is new)

Originally posted by @FingerMove in #447 (comment)