MFA - One Time Passwords

To use one time passwords as an authentication factor, users need an Authenticator app such as:

MFA OTP Signup

Upon signup, they can scan a code and set up the app, upon which it will begin generating one-time codes.

Afterwards, when logging in to the app, the user can simply check the authenticator app for the current one-time code:

Google Authenticator OTP

And enter the code at the prompt:

MFA OTP Login

Your users will need to have an OTP Authenticator app installed in their mobile devices.