Important: You need to enable push notifications for the Authenticator app to work properly.
Go to the app store of your phone manufacturer and download Authenticator. An ad will place a different authenticator app at the top. The actual Microsoft one looks like this:
After that go to office.com, click on your initials in the right hand corner, and select View Account.
On your Account page, select Security Info and then Add sign-in method.
First you will be prompted to enter your phone number. After you confirm your phone number, select Add sign-in method again.
In the drop down box, select Authenticator app.
Select Next on the next screen.
Read this screen. It states you need to enable push notifications for the Authenticator app. Select Next.
Open the Authenticator app on your phone and select the + icon on the right had corner of the app and select Add Work or School account.
Select Scan QR code and hold your phone up to the QR code on your screen.
You will be prompted to approve the login with a two digit code.
You can now close out of the Authenticator app and the browser window.