You will be prompted with a screen to enter your location and your phone number. This number should be your mobile number for the device you are installing the app on. After the number is entered, tap on next.
From there you will be prompted for a verification code. The code will be sent to your number through your default SMS app. I was using Hangouts at the time, and the verification code sent to me was automatically entered into the verification screen. But if that does not happen for you, just enter the code. The application will verify you and you will be all set.
Click the Video Call button to start your first call. And bam, you are now off and Duo-ing.