The Prusik Knot, also known as a friction-hitch, requires two lengths of rope of different diamater. This knot is useful when you have a loop that needs frequent adjustments. Simply release the tension and slide the knot along the master rope using the carabiner. This knot is self-tending and can be used to ascend rope in an emergency situation (such as a crevasse rescue). See also the Bachmann Knot.
Take the smaller diameter rope tie the two ends together to form a loop (using the Fisherman's Knot, for instance). Make a prusik loop (one loop is shown at left). Make additional prusik loops (3-4 total, depending on rope and load).
You'll want to make sure your smaller diameter rope is an appropriate length once the prusik loops have been completed. To use, simply apply the load to the loop and the knot will hold fast to the guide rope. Simply release the load and you can slide knot with ease.