N600 range extender set up on a mac

I had this all set up to work with the Amazon firestick once...had to make some changes and now i cant get it set up again.  I have a modem hooked to an Apple airport express via Ethernet (WAN) with everything else running wireless.  Which of the 3 ways do I use?  Can't use the WPS way as the Airport has no button.  I suspect i have to use the ehternet cable way.  So is this correct?....ethernet to router AND another ethernet cable from the computer to the extender?  I'm really confused and I know I didn't have this much trouble the first time....any help would be appreciated.  The directions don't seem to be Mac friendly.  Thanks