Once generated, the patient will receive an email with the code, or you can give it to them from the "Overview" tab under the patients area.
Once the patient download and installs the application, if they have never setup an account or you need some guidance please see: Setting up your Kardia Account
They will be asked during setup if the have a referral code from their doctor, they can enter it at this time and the app will ask them for their shipping address and payment information.
If they have the App already setup and did not enter a referral code during setup, they can add the code by tapping on 'Profile' at the bottom of the screen and part way down you will see "have a referral code from your doctor?" tap on that and enter the code provided.