how to add an Airport express to extend (WDS) an Airport Extreme network

macosxhints - Airport Express WDS with third party NAT / DHCP: "This hint describes how to add an Airport express to extend (WDS) an Airport Extreme network that uses some other (wired) host (e.g. a broadband router) to provide DHCP and NAT. The Airport Express Assistant will let you set up WDS automatically, but it insists that the master airport uses NAT and DHCP, and has a static IP address, which means you have to form a separate subnet for the wireless portion of your network. "