this instructable shows you how to, well... tie a slip knot with one hand!
Step 1: Step One
start with the the rope laying over the back of your hand like this.
Step 2: Step Two
cross your thumb around the rope.
Step 3: Step Three
what you just did should look like this.
Step 4: Step Four
push the loop down with your thumb then it will kinda look like a pretzel.
Step 5: Step 5
pull the rope through the hole like this.