Pack lots of water. On the Mokulēʻia side, about 1.75 miles past Dilingham Airfield, park at the end of Kamehameha Hwy on the left or right. Make sure to lock your vehicle and not to leave valuables inside. There are no bathrooms or water sources at this spot.
Enter through an opening on the side of the big yellow gate. It will be very bumpy when dry and very very muddy after rain so be prepared for that. Lots of "side adventures" if you decide to venture on the side trails. Again, make sure to only use marked trails so as to prevent further erosion in the area. Do not disturb the wildlife or the flora in the area and do not leave your trash.
Ka'ena is a protected area under state laws and could land you some hefty fines or jail time if not both. The area is patrolled by park rangers/police. Check with Hawai'i DLNR
for exact rules and regulations.
When you reach two big boulders about two miles in, there will be another gate, I'd suggest not riding inside the gate but you can take your bike inside (it's a beautiful area) take the left trail which will be the start of an uphill ride. Do not follow the fence line when it takes a 90-degree right turn, this will lead you to the rest of the same trail but ends up in an eroded area which was unmarked the last time I rode and it's a good 30-50 foot drop on big boulders and water.
If you keep going straight, the trail would become narrow and at one point you might have to carry your bike. You'll see over the eroded part and when you get past that portion you head back down to the second part of the trail and continue riding to the Waianae side of the trail. Hope this helps. Ride safe. Aloha!