This stretchy, versatile cast on is easy to do but difficult to describe in writing.
Pull out approximately 12” of tail for every 20 stitches to be cast on. Hold your left hand, for lack of a better description, like a gun. With palm down, drape the tail over your left thumb and yarn attached to the ball over your index finger. Turn your palm up and keep your hand positioned in that general position. Use your other fingers to hold strands out of the way against your palm, keeping a loop around your thumb and index finger. Holding a needle in your right hand, grab the yarn between the left thumb and index finger from underneath. This is the first stitch cast on.
Next, holding the needle above the yarn, grab the left-most strand around the thumb from the outside (left to right), then left-most strand around index finger from the outside (right to left). Pull new stitch through the loop on your thumb and onto the needle snugly but not tightly. Repeat this step.