It's the international terminal. The one where NWA flights still use and the one that you see Aer Lingus, EgyptAir, Virgin Atlantic, and others at in the afternoon. Go down the escalators midway down the concourse on your left and there is a lovely little pit where people wait to board the bus that takes them out to the airplane which is parked on a remote hardstand. Sorry, I don't know the gate numbers but if you're based at JFK then you know which terminal I'm referring to, and once you're there you'll see the gate numbers. You do not need a boarding pass to go down the escalators. They will list you down there, not at the top because at the top they are usually just airport employees and not Delta employees.
It is often total chaos. If you have the time, get there earlier than usual because if they want to do an on time departure, the bus can sometimes leave that pit 30+ minutes prior to departure time to head to the plane. Sometimes there's only one bus, sometimes two, staggered if so, and that depends on the loads.
And listing is done on the spot with the gate agent using the Northwest Airlines jumpseat XCM form.