Extend an existing Wi-Fi network in AirPort Utility on Mac
You can extend your existing Wi-Fi network with an additional base station using AirPort Utility on your Mac. After it’s extended, your Wi-Fi network uses the existing network settings and broadcasts to a larger area.
Physically connect the base stations to each other
Verify that the power cord is connected to the Power port
on the primary base station and plugged into a wall outlet.Plug one end of an Ethernet cable into the WAN port
on the base station you’re adding to the network, and the other end into a LAN port 
on the primary base station.Note: Throughput improves when a base station connects with an Ethernet cable rather than wirelessly. If you don’t connect with an Ethernet cable, you see prompts to connect wirelessly.
Extend your Wi-Fi network using your Mac
On your Mac, click the Wi-Fi icon
in the menu bar, then choose the base station listed under New AirPort Base Station.Follow the onscreen instructions.
AirPort Utility Set-Up Assistant automatically chooses settings for extending your network.
If you want to set up your network using different settings, click Other Options.
You can also extend your Wi-Fi network by using your iPhone, iPad or iPod touch to set up another base station.
To see the PDF of the set-up guide that came with your base station, search for its model name on the Apple Manuals, Specs and Downloads website.