Practice Before Using
The carabiner brake was the safest way to rappel before the use of belay and rappel devices in the 1970s. It is, however, complicated to set up with six components and can be rigged incorrectly, especially in bad weather or darkness. Practice rigging and using the carabiner brake before you actually use it in an emergency situation.
What You Need
You need six carabiners for rigging. Oval carabiners are best, although D-shaped carabiners also work. Avoid bent gate carabiners unless that’s all you have in an emergency. Locking carabiners are useful. Use a large locking carabiner instead of two regular ones to clip onto your harness belay loop. Locking carabiners are also good for the brake, especially if you want to use a single carabiner for less friction than double carabiners. Auto-locking carabiners are better than screw-gate ones since they won’t accidently open.
Always Oppose Carabiner Gates
Always reverse and oppose all carabiner gates when you rig the system so they can never accidently open. Also don't set up the carabiner brake directly on your harness belay loop. Always use two carabiners or a locking carabiner to rig the brake on, otherwise you risk abnormal wear and damage to the belay loop.
Step 1 to Rig Brake
Take two carabiners and clip them onto your harness belay loop. Make sure the carabiners are reversed and that their gates are opposed so they never accidently open at the same time. Alternatively, use a single large locking carabiner, auto-locking preferred, to clip onto the belay loop.