What your talking about is called Cleaning
Cleaning
This video covers the process in detail.
But in short:
- Attach your self to the top anchor with a spare clip draw (or two), or better yet a sling attached to your harness using a larks foot and a locking biner.
- Inform your belayer that your safe (but not off belay)
- Tie a figure of eight on the bight in the rope
- Attach the rope to yourself using this knot (prevents you from dropping it), use a locking biner for this
- Untie your attachment figure of eight
- Feed rope through the belay loops at the top (should be two)
- re-tie the end of the rope to your harness
- weight the rope to confirm it's attached safe (communicate this too your belayer)
- Once your happy your attched correctly and safe shout on belay to your belayer, they should take any slack
- remove and untie retaining knot on rope
- remove quick draw(s) attaching you to the anchors
- Rappell down
- Remove quick draws as you descend
- Once your on the ground untie and pull rope out of belay loops
Retreating
Retreating is a bit more complex. To retreat you need to leave some gear behind. Best is some cheap (but strong) locking steal binners. You basically do the same as above( but rather than looping the rope through the anchor, your attach the binners to the anchor and link through these (~your rope won't fit through a typical sport anchor)
If your not too high the above is fine. If your high (or want to be safe) this introduces a single point of failure to your system (if that anchor fails your dead).
A good method to avoid this is to loop the rope though the top anchor (using your binners) and the one below it. This way you have a back up to the first failing.
- So loop yourself though the top most anchour, as above.
- Lower off a little to the second anchour
- Attach the rope to this anchor using your second carabiner (do this before you remove the quick draw)
- Remove the quick draw and clean as usual