To do a long tail cast-on you should give yourself at least 12" of tail for evey 20 cast-on stitches. Becareful how tight you pull on the yarn after you make a stitch so you don't cast-on too tight.
Grab both ends of the yarn in your hand and secure with your fingers. Then insert your thumb and forefinger between the strands of yarn and pull creating a upside down triangle with the yarn.
Holding the needle in your right hand insert the needle into the center of the triangle.
Bring the tip up through the loop on your thumb.
Then down through the loop on your finger
grabbing the yarn with the needle
Next pull it through the loop on your thumb.
Let the loop slide off and then bring your thumb back under the loose strand of yarn to tighten the stitch on the needle.
The first cast-on stitch creates 2 stitches after that each cast on will create 1.
Cast-on the required number of stitches making sure not to pull too tightly when casting on.