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 or AirPort Utility for iPhone, iPad, or iPod touch. After extending your Wi-Fi network, your 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.If you don’t physically connect the base stations using an Ethernet cable, you’ll connect wirelessly during the set-up process.
Note: Using an Ethernet cable to connect the base stations provides the best throughput and is thus preferred over connecting 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.
Extend your Wi-Fi network using your iPhone, iPad, or iPod touch
On your device, go to Settings > Wi-Fi, then tap the name of your base station (for example, “AirPort Extreme 0affff”) listed under “Set up new AirPort base station”.
Follow the onscreen instructions.
The set-up assistant automatically chooses settings for extending your network.
If you want to set up your network using different settings, tap Other Options.
Detailed information about wireless networking, including a variety of support articles and a PDF of the set-up guide that came with your base station, is available on the AirPort Support website.