As with many other cast on methods, the long-tail cast on method begins by forming a slip knot.
Instead, you have to start with a long tail, and the stitches are formed using both the tail and the other side of the yarn are used to form the stitches.
Once you've measured out how long your long tail needs to be, form the slip knot.
After the slip knot is secured, hold the needle in your right hand. Position the work so the yarn attached to the ball is in front. Wrap this strand around your left thumb, so it goes counter-clockwise around the thumb. The tail should go over your index finger and be held loosely between the two fingers.