I did this last weekend - really easy.
Take the air train that goes to Federal Circle station (the one I took was described as a "Howard Beach" train) At Federal Circle go down stairs and out the doors. Turn right and follow walkway under tent/awning to where the Hotel Shuttle buses pick up. The Sheraton bus also goes to Hilton Garden (check with driver to be sure) as the hotels are next door to each other.
Hilton Garden