Below is the setup guide for your Airport Extreme
The article below outlines the optimal wireless settings:
If your signal is weak in another part of your home, you may need to extend the network with an Airport Express:
Extend an existing Wi-Fi network’s range.
If you already have a wireless network in your home and want to extend its range, AirPort Express can help. Just place it in range of your primary base station — an AirPort Extreme, Time Capsule, or another AirPort Express — and near the area where you want your wireless connection. Launch the easy-to-use AirPort Utility app on your iOS device or Mac, and you’re mere minutes away from long-range Wi-Fi enjoyment.