ArtM left out a stop sign/turn, and that may be the source of Sarah's confusion.

Approaching from Roslyn, the Custis trail does curve sharply to the left to cross under I-66. At the bottom of the underpass, the Custis trail zero mile marker is at the stop sign.

To get to the W&OD from that point, make a sharp right at the stop sign. Follow the park connector trail, which curves to the left away from the highway, for about 100 yards. It will come to a "T" intersection and second stop sign at the W&OD. The two intersections are not visible from each other. I think that 100-yard park connector is the source of much confusion--it is not officially part of either trail. At the T with the W&OD, right will take you to Vienna, Reston, Pucellville, and California. Left will take you along the Four Mile Run and, after some street riding, to Reagan-National.

Coming from the West, along the W&OD, there are no stop signs. You would make a sharp left on the park connector, curve around to the right, and another left at the start of the Custis trail, going under I-66. The first time you do this, you will think,
I
should
have
down
shift
ed.

Happy riding!