Rappelling, or abseiling is a vital part of rock climbing. What else would you do after reaching the top of a climb? You have to be able to get back to the bottom safely. Rappelling is the safe method chosen by climbers when there is no easy way to climb down and no trail to get back to the bottom.
To get ready to rappel you have to prepare several things: the anchor, your harness, gear and the rope for other rappelling methods.
THE ANCHOR has to be bomb
proof (extremely safe) since all of the person's weight will be placed on it. At least two
independent anchor points should be used when setting up anchor. Select strong and sturdy
objects to secure your rope, do not use saplings or large boulders that can be moved. Use
well developed trees or rock formations. Attach a sling to each anchor point and connect
them with a loop. There is no need to use a carabiner since all equipment will have to be
left behind. Back up the anchor with a chock. String the rope through the loop at the end
of the anchor slings using the rope doubled. Place the rope in the descender you are
using. These pictures only have a single rope but double rope is essentially the same
except that instead of one, two ropes are used.
When rappelling, keep your feet
in front of you when on a straight face. When on a slope, just walk down slowly backwards.
The friction of the rope against the belay device and the climber's grip slows down the
rate of descent. However, if a belay device or a harness is not available, then you can
use the Dülfersitz rappelling method. This method is used in emergencies, is very
uncomfortable, and has to be done slowly since fast rappelling can cause friction burns. 
The Dulfersitz.The rope runs between the climber's legs, under the right thigh, diagonally across the chest and over the left shoulder, then diagonally across the back to the right hip where it is held in the right hand. People who are left handed do it the opposite way. To brake, lift the right hand into the horizontal position. Look at the picture--it is easier!