I have two Buffalo Tech WHR-G54S Router/Access Points.

One is set up as a wireless router. DHCP on, wireless enabled, SSID
changed from default. The WAN port is plugged into my Ambit cable modem,
my computer is plugged into a LAN port. I have the internet (this is
coming through it).

The second WHR-G54S is to be an access point upstairs. I have tested
it hooked up to a LAN port on the first router, and it works.

What do I need to set between the two WHR-G54S so that I can set the
second up as an access point upstairs?

SSIDs should be the same
Security on both needs to match (in this case AES)
DHCP on downstairs, DHCP off upstairs.

Anything that is missing?