Long tail cast on

From BluWiki

This is one of the most popular and versatile cast on methods. You start by pulling a length equal to three times the width of your finished project. So if you want a six in wide scarf, pull a tail just over 18 inches. Form a loop. Pull the yarn behind the loop. Reach in through the loop for the working yarn and pull it through. Put that on the working needle, and pull it, but not too tightly, effectively making a slip knot.

Insert your thumb and forefinger between the V formed by the working yarn and the tail. Hold both strands with your other three fingers and pull the needle down and back a little.

Insert the needle into the loop on the thumb, coming in from the outside.

Pick the loop that is around the forefinger.

Pull that loop forward and through the thumb loop.

Take your thumb out. Pull the loop closed, but not too tightly, and return your hand to the starting position.

The video demonstrating this technique is here.