Take 101 north, take Hwy 1 off ramp. Go back under the freeway. 

From East Bay:

Take Richmond bridge north then 101 south. Take off ramp to Hwy 1 (Mill Valley/Stinson beach).

Then for both ....

 Turn left at the traffic lights at the Gas Station towards Mt Tam/Stinson beach (ie away from Mill Valley). Drive several miles until there are no houses, then take the right turn on Panoramic Hwy towards Mt Tam/Muir Woods. Go another mile or so until you get to four corners - take the road straight (up the hill towards Mt Tam). Drive another mile or so and then go left  Ridge Avenue (just past the last of the houses).Drive down the little road to the parking lot. 

Need a medium sized map?
Still lost? Here's an even bigger map.