Everyone should know this method. It is easily rigged with a few carabiners, and does not require a special descender. It is generally safe (at least by rappelling standards).
The two vertical carabiners should be oval carabiner set with the gates up and opposite. D-shaped carabiners can be used, but will bind if the gates are opposite, so they should be rigged with the gates on the same side and opposed. This is not quite as safe in theory, but in practice no one seems to notice. The cross carabiners act as brake bars. They can be ovals or Ds. In a pinch one will suffice, on the other hand, one can get more friction by adding a third cross carabiner.
The rappeller should attach the assembly to the seat harness with a maillon (preferred), one or two locking carabiners (an HMS carabiner is easiest), or two non-locking carabiners. Whenever two carabiners are used, the gates should be opposite or opposed.
This method takes a little bit of time to rig, but it is simple to verify that everything is correct, it gives adequate friction, it does not twist the rope or cause the rappeller to spin, and will probably never come off the rope by accident. The disadvantages are that any sandy mud will soon wear grooves in the carabiners. This may not be a concern to climbers, but for cavers who often rappel in mud, the financial impact can be severe.
The rig can be doubled by piggy-backing a second set of carabiners with brake bars. I've found that this creates quite a bit of friction, which is nice on some climbing ropes but will prevent motion on dirty stiff caving ropes.
There is an alternate method that I mention in passing. The
rope can be brought out above the carabiner gate, and passed over
the shoulder as in a Dulfersitz rappel.