Asterisk newbie, not sure how to assign an extension/user to a device (using analog phones)

Dear all,

I am asterisk/freepbx newcome, and I am trying to create a pbx. I have inserted the fxo/fxs cards and I have analog phones connected to the ports on the fxs card. I plugged the pstn into an fxo port. (However this was just a test and error because I am unsure as to what ports are assigned to the devices/pstn. I also created extensions on freepbx. I know that the help that I am asking for is broad, but any insight would be much appreciated. I want to at least be able to call from one phone to another so that I know that I am on the right track.