What you needHook
The slip knot is a simple knot that -slips-, the loop easily enlarged or made smaller by pulling either the loop to enlarge the loop, or one of the yarn tails to make the loop smaller.
Start by holding the yarn between two fingers, with a moderate amount of tension, not tight, but not so loose that you lose your grip. With the fingers of your other hand make a loop of yarn with a single twist.
Now pull one side of the yarn through the loop you just created making a second loop.
Take hold of that second loop and hold it while holding the tension of the yarn somewhat firm.
If you can pull the loop and make it bigger, then pull one of the yarn tails and make it smaller you have a slip knot.
(Note: only one yarn tail will make the loop smaller, the other one is stationary).