Unzip the armrest cover and pull it at least halfway off (you might have to also slip the padding down - can't recall) to show the large bolt (remove it with a #50 torex socket) that holds on the entire armrest assembly. Undo the bolt and the arm comes off.
This isn't an Expedition armrest, but it's close... the 2nd picture will give you the basic idea:
http://www.f150online.com/forums/articles-how-tos/416301-1999-manual-lumbar-fix.html
If you are leaving the armrests off permanently you first have to sell the armrests to me, then you have to cut off the stop pins (again, linked picture is not an Expy seat, but close):
http://www.ford-trucks.com/forums/861697-bucket-seat-arm-rest-removal.html
The stop pin on your seta is about 3/8" thick, about 1/2" long and made of steel. It can be cut off with a hacksaw but it's easier to cut off using a high-speed air cut-off tool if you have access to one. Be sure to file down the stub of the pin to remove sahrp edges or it'll cut through the side of your new seat covers.