Can anyone point me to the document which details setting up an extended network?

I have both a current gen Airport Extreme and Airport Express. Up until a few weeks ago, I had successfully extended my network within my house, and then screwed it all up. The Extreme is still working as the primary, but I can't find the document which details exactly how to extend the network to the Express. I seem to remember an install document which had visuals of where you have to physically connect (via ethernet cable) the Express to the Extreme during setup - but can't find it anywhere. Can someone point me to this document?

Thanks!