There will be a little video on the flight (assuming it's united) outlining the process as they are starting to come into DC. I don't think there's any audio on it, I may be wrong, it will stake them through with the animated people.

Also tell them all the stupid stuff if they do not travel that often, fill out the forms on the plane, have their passports with them so they don't need to dig them out of their carry on, etc.

It's a fairly simple process to clear at IAD and get back up for another flight.