Pretty easy, The Narrows are at the end of Floor of the Valley Road in Zion NP. During the busy season you have to take the shuttle in as they restrict auto use.
My first time in the Narrows of the Virgin River in Zion NP. Amazing place and amazing photography everywhere. We timed it nicely as the glow of the bounce light off the canyon walls was incredible. I just recently discovered this shot and hard to believe I'd missed it many times looking through the images.