This tutorial has many names: Sliding loop technique aka Slipknot for circle ring aka Magic ring.
With the long side of the yarn in the palm of your hand, wind yarn around index finger twice.
From right to left bring hook underneath the loops on your finger and pull the yarn on the left end through the others. Yarn over and chain one. Carefully take out finger from the ring, chain the no. of stitches that is asked of you.
Now for the fun part! You will see you have TWO circles and a tail of yarn hanging, what you want to do is first make sure which of the two circles moves when being pulled, work your stitches and then FIRST completely pull that moved circle (which makes the other circle disappear) and then PULL the tail of yarn and see how the magic is done! A perfect circle is made! 🙂