Every Setup is different you can find guides to each Gateway right here inside the Documentation.
The general process is:
Activate gateway from the Store site
Enter setup parameters
Devices appear in your device list
Done! You can now map your newly added devices to existing Apps or create new ones
Tips:
To map the device you need to add hardware to your application. Inside the debug overview that you can reach from the device view you can find out what hardware your gateway device comes with.