In the illustrations below, blue indicates the working yarn, held with the thumb and wrapped over the top needle. Red indicates the tail, held with the forefinger and wrapped over the bottom needle.
Position the 2 tips of a circular cable pointing to the left, and wrap the yarn around the bottom needle like so. (This will become your first cast on stitch.) Hold the working yarn with your thumb. Hold the tail with your forefinger.
Step 1: Cast on the first top stitch. Bring the working yarn up behind the bottom needle and in front of the top needle.
Now swing your hand down behind the needles, as shown by the arrow. This will bring the working yarn over and around the top needle. The result will look like this. You have now cast on two stitches: one on the bottom needle, and one on the top needle.
Step 2: Cast on the next bottom stitch. Bring the tail up in front of the bottom needle and behind the top needle.Step 3: Cast on the next top stitch. Bring the working yarn up behind the bottom needle and in front of the top needle, just as you did in Step 1. Swing your hand down behind the needles... ...and you will have two stitches on each needle. Repeat steps 2 and 3 until you have cast on the desired number of stitches. Then you can continue to knit with your working yarn.