One of the more useful knots for climbing is the Purcell prusik, which can be used as an adjustable tether or for ascending a rope among other things.
How would one tie it ?
Note: You want to keep the double fisherman's knot close to the opposite end, but offset slightly, this gives you maximum adjustability while being able to use the far end as a prusik as well if necessary.
There are two ways I have done this,
Tie a normal prusik (described here) around something and then pull the end of the cordelette through. A stick might work, I have always just used my arm and then pull the end through with my hand.
The difficulty with this technique comes from getting the prusik over your wrist as it has a tendency to tighten.
The other is to tie a prusik on a bight and then pass the end of the cordelette through.
It should then look like this,
From there you add at least one extra loop to each side, although I usually do two, and then pass the end through.