There are all sorts of ways to make a slip knot.
If you have a crochet hook, try this one: Wrap the yarn around two fingers, crossing it at the front and holding down with the thumb.
Now insert the crochet hook under the right hand thread and over the left hand thread.
Draw crochet hook through to the right. (You do not need a crochet hook – you could just reach under with your finger and thumb to draw the thread through.)
Slip loop off your fingers holding both ends with your left hand.
Now simply tighten the knot and there you have it.
Or watch this video to find out a few more methods of making a slip knot