1 Measure out several inches of yarn and hold the yarn as in the picture above -- the "tail" (the yarn that doesn't go to the skein) over your thumb. A good rule is one arm's length for every 50 stitches you want to cast on

2 Pull the needle over the top of the yarn running from your thumb to your index finger, then pull it down in front of the thumb.

3 Put needle up the thumb making sure that you go through the "loop" around your thumb. If you aren't certain, touch the tip of the needle all the way along your thumb as you go up.

4 Reach over and grab the yarn from the index finger.

5 Pull it back down and through the thumb. 

6 Let your thumb "out", while firmly holding the yarn with your other fingers. Re-insert your thumb under the tail yarn and firm up the stitch around the needles. With this method you always get TWO stitches with your FIRST cast-on, but after this only ONE will be cast on at a time.

7 You won't need to come back down over the top of the yarn, as you did in step 2, because your yarn is established.  Go up your thumb as in step 3, and repeat.