This is a popular hike, so expect a lot of hikers, particularly on weekends.
You can park at Malibu Canyon State Park ($12 in 2017; they take credit cards). Malibu Creek State Park is the former 20th Century Fox Ranch which was used as a set for several films.
From the parking lot, the forest road follows Malibu Creek as it winds through the park. Crags Road actually passes through the creek, but you can take the High Road
bypass as the creek may be too deep and/or covered in algae to ford if it is not dry.
Shortly past the visitor center, you'll pass Century Lake. At the far end of Century Lake, there is a bridge over Malibu Creek. Take care as there is a steep, two-foot drop on the other side of the bridge. It's rideable if you don't want to jump it, but it may catch you off guard if you are not expecting it.
After the bridge, the trail becomes a singletrack paralleling Malibu Creek until you reach the M*A*S*H site. There are a lot of large rocks in this area that you'll have to steer around and particularly if there are hikers on the trail, it's likely you'll have to dismount a few times along the way.
The M*A*S*H site will probably be underwhelming for many people. None of the buildings (which were tents) from the show are present, and many people are probably too young to remember it now. There are a few rusted vehicles and one 50's-era Army ambulance that is in better shape.
Beyond the M*A*S*H site, Crags Road again widens to a forest road. It ends at a housing area, becoming Crags Drive. Or you can climb Bulldog Motorway
(if you really like climbing) which takes you to the crest of the Santa Monica Mountains for some awesome views.