Go all the way down H1 until it ends by the Air Force security gate past Waianae. enter the gate with ID and the DLNR hiking pass in hand (good for 2 weeks) or a Hawaii hunting license (good until license expires). Go all the way to the top where it "T" intersects by a dirt parking lot and another guard shack. Hang a right and just keep following the road until it comes to another dirt parking lot and hang another right where it will turn into a dirt trail for the remainder of the scenic route!!
As you get toward what appears to be the top you will see a jeep sign pointing to the left. if you follow that way it will take you a couple to few hours to get to the Peacock Flatts Camp Grounds at the end of the trail. If you go right, it will take you about an hour to 1.5 hours but you get more "top of the mountain" scenery along the way overlooking the shore lines.
There are several "splinter" trails leading off the main one that can be a little "edgy" due to not much room before you roll off the mountain and several guys more recently have flopped their jeeps on them due to DEEP ruts from mountain rain run-off.
The only other legitimate off-road area is Kaena Point, as well as the SandBox 4x4 park at Sand Island when they are open.
How to get your DLNR pass for P-Flatts:
Be sure to fill out and fax into the Honolulu Office. if you send it early on Monday through Thursday you could have it back by the very next business day in your mailbox stamped with official seal.