Sombrio Beach is the beginning of the Juan de Fuca trail. From Sooke, head west on Hwy 14 for 55.8 kms towards Port Renfrew. There are roadsigns on the side of the road letting you know Sombrio Beach is approaching. The road in is only about 1 km of unmaintained, rutted road. We easily accessed the beach in our campervan, though, so most vehicles can definitely navigate around the potholes.
Once you arrive to the parking lot, there's a well marked trail head. Stay left at the first fork, and then walk along the beach for about 20 minutes before you run into a creek. There, you'll head up the creek bed (maybe 200 m) and you'll reach the entrance to the Slot Canyon Waterfall.