http://superuser.com/questions/122441/how-can-i-get-the-same-ssid-for-multiple-access-points
http://support.apple.com/kb/HT4260
In this image, #1 is the primary WAP and the #2's are secondaries:
Try to use cabled Ethernet to connect the primary wireless access point (WAP) to the secondary WAP units
If you can't use cabled Ethernet, use a separate wireless network for the uplink from the secondaries to the primary
If you use a separate wireless uplink, use a different technology
802.11b/g/n for clients
802.11a for uplink
Enable
NAT on the primary WAP/router that connects to the Internet
Optionally enable the
DHCP Server function as well
Only one
DHCP server per subnet
Don't use routing or
NAT on the secondary WAP units
Use the exact same settings on all wireless access points
SSID
Security (WPA2-PSK)
Password/Pre-Shared Key
Set each WAP to a different channel