Tight switchbacks provide plenty of challenge going up and down. There are stream crossings and some steep cliffs to be aware of. Every time I start to feel like I'm getting in shape I go ride Hau'ula to humble myself.
With that in mind it's a beautiful ride worth doing, even if you walk much of the way. The downhill is worth the effort of the climb. As long as you know how to duck under low hanging trees.
Heading from 7-11 you ride north to Hauula Homestead Road. Turn left towards the mountains and ride all the way to the end of the street. Hauula Homestead Road forks off to the left but go straight ahead. You will run into a yellow gate that you can ride past. 30 yards further you will run into another gate, no need to dismount, just ride around it. You will ride on a paved trail for a hundred yards or so before you see the signs for Hauula Loop Trail on your right.
Once on the trail you soon come to a small creek to cross. Get past this and start your climb to the fork in the trail that begins the loop. Either way is ridable. Riding clockwise leads you to a pretty intense climb with a long mellow downhill. Riding counter-clockwise gives you a longer but more mellow climb with a shorter, technical downhill.
Pedestrian traffic is pretty much limited to locals hiking the trail for exercise and is fairly light but always keep on the lookout.
There is not much parking at the trail head. Parking on Kamehameha Hwy near 7-11 is probably the best bet. It's only about half a mile to the trail from there.
You may want to add some miles by riding the adjoining Ma'akau Ridge Trail while in Hauula.
Clicking on the link above you bring you to Google Maps where directions to the trail are plotted in from Honolulu Airport.
Download the GPX file and use your favorite software to upload trail data to your GPS. Right click on the link and select "Save Link As..." in order to download it.