US Outsdoor Maps and Books blog has just published excellent directions including a video for hiking up to the Hollywood Sign, which is on Mt. Lee and not on Mt Hollywood for whatever reason.
Just remember that it is illegal to go beyond the fence. Historically, park security officers have little sense of humor about this particular bit of trespassing if you choose to ignore the warnings. Why? Because the communications towers on Mt. Lee are a major Homeland Security site.
No one has a sense of humor when it comes to Homeland Security dealings.