AirPort Express to share ethernet client to give another router internet connection
I have an Airport Extreme which is connected to the internet via my cable modem. Across the house is my home theatre which has a few wired cameras and older computers. I purchased the new AirPort Express so i could use ethernet clients. All AirPort Express wirelessly to my existing wireless network and then share the ethernet connection which i will then have a patch cable running to the uplink of another router. Im wondering if there is a specific way i need to set up the other router and also the new AirPort Express?
MacBook Pro, Mac OS X (10.7.1)