Slip knot
My best explanation:
For me, (right handed) I place the tail of my yarn down my yarn drapes over my fingers, then, pull it round the top two fingers, so it drapes over the top once more (so its entirely gone round my top 2 fingers), but on that the end with the ball on it is to the left of the original, so it causes an X on the front of my fingers, then I stick my fore finger of my right hand or a hook through my 2 fingers and grab the long tail and pull it through. This makes a loop.
Another way is demonstrated above
