1. Step: Install AquaMail (free version) from Play Store
Then start AquaMail once and configure one of your email accounts (enter your login data)
Close the app
2. Step: Buy and install AquaMail Pro Key (unlocker) to activate the free app to “pro” status.
3. Step: Start AquaMail (the "normal" app icon, not the "Pro Key")
Enjoy the Pro features