how do i sign into youtube with the youtube app using my pre-existing youtube account?

Download the YouTube app to stream online videos on Xbox 360

Sign in with your Xbox Live gamertag.

Go to apps, and then select Browse Apps or Search Apps.

Browse or search for YouTube.

Select YouTube, and then select Confirm Purchase to download and install the app. After the download, the app automatically starts, and you can browse the available content.

To start YouTube later, select My Apps, and then YouTube. You can also start YouTube by selecting Video, and then My Video Apps.

If you have a YouTube account, you can sign in to your YouTube account and access your YouTube subscriptions and playlists.

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Youtube app is rubbish compared to the 360 one, why the step backwards? Also when I am using the youtube app and watching a video if my son walks into the room it'll sign him into the xbox one but also cut out my video I was watching and sign me out of youtube
and sign him into youtube?

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That can't be right can it? Just because someone signs into the xbox one it should not just switch user on the app I am using unless I sign out, games I play don't do it?

Does it say this on the YouTube screen when you launch the YouTube app? Are you also signed in to the YouTube account that you have already?

youtube and the youtube app is owned by google, you want to watch it you have to sign in or link the youtube app to your google account, using netflix requires you to login and it auto registers the app your using to your account, like with many other apps,
if you don't like it then don't use it, if you watch youtube on the pc you still have to login to prove your age which then registers your pc to your google account.

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Assuming I understand the internet translation correctly,and you're on Xbox 360:

To use the YouTube app, you need:

• To be signed into an Xbox Live Account
• At least 83 megabytes (MB) of available storage space
• High-speed Internet access

To "Confirm Purchase"

1. Sign in with your Xbox Live gamertag.
2. Go to apps, and then select Browse Apps or
   Search Apps.
3. Browse or search for YouTube.
4. Select YouTube, and then select Confirm Purchase to download and install the app. After the download, the app automatically starts, and you can browse the available content.

After the above steps if you have a YouTube account, you can https://support.xbox.com/apps/youtube/account-sign-in and access your YouTube subscriptions and playlists.