Know Before You Go
You will need a SUV with clearance to get here, but four-wheel drive is optional. A guide/tour is not needed. You should be familiar with driving on dirt roads that are heavily potholed, as it is easy enough to pop a tire if you drive too fast and hit one of the rocky potholes. The drive takes between an hour and a half to two hours from Riyadh.
On The Way Around lists two different approaches. The most popular one is via the Acacia Valley, but it is only open on Fridays and Saturdays. The other approach via the Sadus Dam is open at all times. Roads are rough on both routes, and low-rise vehicles will have trouble.
The Google Maps way has no signs or much in terms of physical waypoints, but the GPS is spot on. MAPS.ME also has accurate GPS for the roads.
Be sure to stock up on food and water before attempting either road. It's best to go with another vehicle in case you get stuck as there isn't any cellular reception here.