Parking is easy, it's across the street from Castle Hospital at the park with the fountain. Refer the Google Directions below.
You will be riding through some neighborhoods at first, then up some steep back road until you arrive at the Pali, Kamehameha intersection. Cross the intersection and head up the Pali taking the first right. You will soon reach a gate/fence for the Old Pali Road. Keep climbing until you reach the Pali lookout.
At points on the Old Pali Road the greenery is so overgrown it feels like single track although the ground is paved. It's fun to ride under the Pali highway too. At one point you climb a little ladder and are about 10 feet away from the cars screaming by on the Pali. I thought it must be kind of funny to be driving down the Pali Hwy and to see random mountain bikers popping out of the cliffs on the side of the road!
Later in the ride you will be looking down at the cars driving by on the Pali Hwy as well has have the entire Windward side of the island in view. It's spectacular. Keep riding till you get to the Pali lookout and enjoy when all the tourist tell you how amazing you are to have ridden your bike up that far.
From here it's pretty much all downhill except for one climb right before the park where your car will be.
I would recommend this ride to most people. Be prepared to get a workout, it's a long steady climb. Not really technical and relatively safe. There are some odd sections where you have to duck under some low hanging freeways :) climb a ladder with your bike etc, but overall it's a nice mellow, enjoyable ride.
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.