If you're the owner of the new account, there are two ways to start using it. You can click the administrator's account name at the top of the Start menu, which displays all the accounts on the PC; then click your account name and log into your new account with your Microsoft account password. Or you can also restart the PC -- the Windows login screen will now have two accounts on it -- yours and the administrator's. You can click your account name and log in with your Microsoft account password.
Your account name will now appear at the top of the Start menu. When the account name is clicked, it will show all of the other accounts on the machine.
The account will have its own separate OneDrive storage. Each user's local files and folders won't be accessible to the other. You will see the Windows apps you've installed, but not anyone else's, and vice versa. Given that it's a Microsoft account, files and settings will sync with all your other accounts as well.
Creating a new user account without using a Microsoft account
Creating a shared PC if the other person doesn't have a Microsoft account takes a little bit more work.
To do it, go to Settings > Accounts > Family & other users > Add someone else to this PC. From the screen that appears, click "I don't have this person's sign-in information." Then at the bottom of the next screen, click "Add a user without a Microsoft account."
Now you can add a user name, a password and a user hint. The person's account name will now show up the same way as if you had created one using a Microsoft account.
When you create an account for someone in this way, it's called a local account. Unlike a Microsoft account, it won't sync settings between devices and it doesn't give access to OneDrive. In addition, when someone has a local account, they won't be able to download or install any apps from the Windows Store. They won't be able to install desktop applications, either -- when they try to install them, they're prompted for the password of the administrator's account. So an administrator will have to be nearby to type in the password in order for them to install desktop applications.
Switching between accounts
Once your account has been set up, it's easy to switch from the account currently running on the machine to your own. If someone else is currently using Windows 10 on the machine, and you want to use it, you can click the Start menu, click the account name at the top, then click the account to which you want to switch and type in the password.
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