Go to terminal A (take the Airtrain if you're coming into a different terminal) and walk out of baggage claim level towards the employee buses. At the end of the line of buses, there will be a FedEx employee bus that will take you to the sort facility/ops building. Easy.
Same as above but for UPS take the employee shuttle going to the F parking lot. Tell the driver when you get on that you need to go to UPS. Only the ones from Term A will drop you off, not C. It'll have a red A marked.