BWI - Take MARC or Amtrak to Union Station. Gally shuttle picks people up on the west side of the station.
Another way is Metrobus from BWI to Greenbelt Station then Green line to Fort Totten, transfer to Red Line towards Shady Grove go 3 stops to NY Ave- Gally station. Shuttle picks up on M street side right outside the doors.
DCA Washington National (the locals don't call it Reagan) - Take Yellow line towards Mt Vernon Square. At Gallery Place transfer to Red Line towards Glenmont. Take red line to NY Ave Gally station. Shuttle picks up at M street side just outside the doors.
DCA is easier, but BWI can work too. IAD Dulles also can be done if wanted.