First, you need iOS 8 and Yosemite on all appropriate devices.
After that, I don't think you really have to setup or activate anything. On my MBA, I open FaceTime and can place either FaceTime or regular audio calls from it. Same with an iPad. Only catch is your iPhone has to be on the same WiFi network. So I don't think it would work if you're out in a park or somewhere without WiFi.
SMS relay I believe will be switched on in iOS 8.1. But I believe it will work the same way using the Messages app.