Create a slip knot about 24 inches from the end of the yarn as shown in these photos.

Step 1.
Place the loop onto one of your needles and tighten.

Step 2.
Place your index finger and your thumb between the two pieces of yarn as shown.

Step 3.
Grab the pieces of yarn with your other fingers below your index finger and thumb and spread your index finger and thumb apart.

Step 4.
Notice that you have created two "loops" of yarn, one around your index finger, and one around your thumb. Send your needle down in front of the "loop" of yarn on your thumb.

Step 5.
Then push your needle up from below through the "loop" of yarn on your thumb.

Step 6.
Now send your needle to the "loop" on your index finger and push the needle through the "loop", this time from above.

Step 7.
Take your needle back down to the "loop" on your thumb and go through, only this time, go through from above the loop.

Step 8.
Carefully slide the "loop" off of your thumb. This will leave a hanging loop of yarn.

Step 9.
Reposition your thumb inside the second piece of yarn coming from the grasp of your lower fingers. Use your thumb and index finger to tighten the "loop" onto the needle by spreading them and the yarn pieces apart.

(Picture shown here has the slip knot and two cast on stitches)
Now you are back to the same position as you were in step 3 and ready to repeat step 4 through step 9 for another cast on stitch.