How do I connect and expand my wifi coverage with a Motorola SBG6580 cable modem/router, Airport Extreme and Airport Express?

I currently have a Motorola SBG6580 cable modem/router with a wifi range that does not reach to the back of the house. So I bought an Airport Extreme and an Express. Can someone show me step by step on how to use the Airport Extreme as the Base Station with ethernet cable connects to the SBG router and use the Airport Express as the wifi extender?