As for any other sport, beeing able to anticipate will be the key to success. Learn it slowly and cautiously. Slow down on the trails when you meet with pedestrians and be friendly under all circumstances. You will create surprise and admiration. Take the opportunity to show the disability on its best aspect...

Beginners

It is best to start on a dirt road with a moderate slope. As soon as a minimum of self confidence is reached and when the beginner is able to safely use the brakes, one can go for some steeper and more technical trails. A close protection from someone walking near the rider might be appropriate. You might later start going for routes that you will choose from the available information, such as this web site, maps, etc..., but always with a minimum of 2 buddies coming along with you.

Reaching the top of the ride

Usually done by car. Paved or dirt roads  give access to many places. Lifts or cable cars are sometimes running during the summer time (July and August). They provide an excellent access to high altitude, which translates to great vertical drops...
When there is no mechanical means available, the human power can be used. One or two tow ropes. linked to your friend's bikes, under their saddle. Pushing by the 2 side ears of the "Cobra"or "Dahu" is also extremely powerfull.
And in many remote countries, animals can be found and can tow you. Horses, mules, donkeys, yacks, camels, you name it... Using a quick disconnect device is absolutely mandatoring. Some of those animals have a strong independant personality.

Switchbacks

Watch your speed. Slow down in time to safely handle the sharp turns. Advanced riders will activate their rear brakes such as to slip the rear while maintaining the front where they want to go. Never forget that switchbacks in steep slopes always translate to high risk of rolling sideways.

Falls

Some more radical techniques 

Narrow trails

Wheelies          

Jumps