The Subway requires a permit from the visitor center in Zion. Your best bet is to enter the lottery that they hold. From the trail head you go down the face of the canyon wall before hiking about 4 miles upstream to this location.
A portrait view of the main chamber in the Subway along the Left Fork of North Creek in Zion National Park. I wanted to get the green pool, glow in the rocks, and fall colors to align in this image.