On your iPhone, Launch the iTypeFastR app (using the icon on the iPhone startscreen), and
   go to 'Purchase & Acticate'. On that screen, you'll find the 'payment code'

  ...fill in the payment code in the field undereath

   Payment Code :

   1 X iTypeFastR License for 1 device = USD $ 3.99

 I read and agree with the End User License Agreement

PS Your license is linked to this device. Once your device is activated, you’ll be able to use the app, and will be even if you restore your device or upgrade your firmware.