Easy bro don't be afraid.. Just get a car and go, can be simple car no need for 4x4.
Take the directions shown above by someone else...
To complement I should say..based on googlemaps for easyness..
Take jeddah road (which is mecca road, the same).... Pass the escarpment as mentioned. To know that you are close you will pass first a check point, which is recognized by the constructions at both sides of the road, like two towers and in the middle a big hand with a sword. Go googlemaps (or google earth) and is easy to find just following jeddah road.
After this comes a downhill pretty cool btw, this is what they refer as passing the escarpment. You can make a stop before or after (just see some simple exits, dont be afraid are no roads just parking spots, you can come back to main road easily) to take pics, if you wish.
Passing the escarpment keep going, like bout 10 minutes or so until you see what they called the Oosar al Muqbel sign. In googlemaps you see it as a clover-shape" intersection, easy to see is the first and maybe only one in many miles. So in clover shape "bridge" you take the second exit..you will be sure because huge sign says "Oosar.... turn right" so keep going and don't mind of the curvature of the road ...you will have confidence when you see a huge sign that says "Nassah" (or smthg like that I forgot, other posts have the same name", it shows that you have to turn right so you follow it and turn right and then go straight ahead until some sand starts to appear...if you see sand you immediately will start to see quads at your left. Dont stop in the first quads keep going until the sand have cool hills and stop there...
Rent the quad, bargain... ask for 80 or 90 SAR no more for one hour. if you make to 50 is ok also
. Then just go to the top hills.. If they don't allow you to go then say sorry i will look for other bedouin...
.... anyways just go even if there is a fence pass the fence, because the fun is in the top hills. just get lost in the top at high speed
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And to make it easier...just go to flickr grab some cool pics of this place and in the pics you see the GPS coordinates, tap the coordinates on your phone and is done