From the L.A. and Orange County areas: take 5 or 15 South to 76 East to 79, turn right to 78/79 (Santa Ysabel) turn left to Julian, OR take 5 or 15 South to 78 East to Julian.
From the San Diego areas: take I-8 East to Highway 67 (toward Ramona).  67 turns into 78 in Ramona, follow to Julian, OR take I-8 East to 79 (through the Cuyamaca State Park) to Julian.
From the Carlsbad / Vista areas take 78 East to Julian.  You can pick up 78 East from I-15 in Escondido or I-5 in Carlsbad.