Take a piece of cord and make a double bight about a third of the way up from one end. Using the bit that dangles off the "up" bight, form a loop and begin wrapping the excess cord around the double bight. When you reach the end (of the bight or the cord), shove the bitter end under the "up" bight loop and pull to tighten the bitter end in the bight.

The first, original throat capo. And the first knot I learned to tie after I learned to tie my shoes.