Hang on to one end with each hand. Do pull-ups if you like. (You can also use two towels if you’d like to space them apart. Throw one towel over a pull-up bar, and hold both ends with the same hand.

While holding the rope in your right hand, grab it between your thumb and forefinger. Now bring your right hand to your left hand so the tips of your thumbs meet (and the newly created loop should ...

Do the same with the rope in your left hand, pass it over and under the rope you're holding with your right hand. Tighten it by pulling both ends of the knot at the same time.