Login to the Office 365 Admin Center: Portal.office.com

Select Admin

Select Users

Select the user you wish to enable multi-factor authentication for.

Click "Manage multi-factor authentication"

In the new window that opens you can either select all users using the check box or select a single user by using the check box next two the users name. Select the user click enable.

Then send the user or users this link> https://aka.ms/MFASetup or you can have them navigate using the web browser installed on the computer (e.g. Internet Explorer, Mozilla Firefox, Google Chrome, Microsoft Edge or if you are using a Mac Safari.) to portal.office.com to start the user side configuration.  After you send them the link click  "enable multi-factor auth"

The Admin side of enabling "multi-factor authentication" is now complete. 

User Configuration

The user will need to use the link provided above (The one sent to the user by you) and sign in.

Once they login they will see "Your admin has required that you set up this account for additional security verification" Click "Set it up now"

They will need to enter the phone number they wish to use for authenticating the account. We recommend using the users cell phone for this. We also recommend selecting "Call me" (Some phone providers do not receive text messages from verification methods.) Then click next.

The user should get a phone call from a number and they will need to answer this. It will then Prompt them to use the # key to confirm.

Once the account has been confirmed they can also copy the "App Password" they will need to use this if they have email on a phone or on outlook.

They can then hit done and the setup is complete.

If you need to change the app password, make sure you are logged into their account and go to this link: https://account.activedirectory.windowsazure.com/AppPasswords.aspx