From Paso Robles at US-101, take CA-46 east 15.6 miles and turn right onto CA-41 West Centre street. Go 2.8 miles and turn right onto Toby Way. After 0.3 miles on
To continue to Carrizo Plain National Monument, travel east on CA-58 24.2 miles, turn right on Soda Lake Road and travel 13.7 miles.
There's a creek at the north end of the road which can flow over the road after a heavy rain, and this can close the road. When I last visited, there was a car in the creek, just downstream of where it had washed off of the road. Don't underestimate how deep the water is, or how little force it might take to push your vehicle off the road!
For more information on the area, I cover Shell Creek Road on page 177 of my "Photographing California Vol. 2 South" landscape photography guidebook.