Park at Mile Marker 16 of Northshore Road. Essentially you follow the dirt road from the parking lot all the way to where it drops steeply in to the wash (about two miles). This section is driveable in a high clearance vehicle.
From there, take the wash upstream to the narrows.
Head out to the lake from Las Vegas. Taking either Lake Mead Blvd or Lake Mead Pkwy, pass the fee booths and turn LEFT on to North Shore Road. The trailhead is at Mile Marker 16 on the LEFT side of the road.
36.1963823271 degrees / 36° 11.783' / 36° 11' 46.98"
-114.6874594688 degrees / -114° 41.248' / -114° 41' 14.85"