I've done this several times. From Crystal City, you talk the path under the railroad tracks that connects to the Mount Vernon trail. You head south along the trail and turn left on at the sign (intended for cyclists) that says "Washington-National Airport." This path winds a bit to avoid the road, goes under the airport return road, and ends at the B/C parking ramp where there is a bike rack. Once in the parking ramp, follow the signs to the exit which puts you at the north end of the metro station, closest to Terminal C.