The Long-Tail Cast On is one of the most common and popular cast ons. It is an all purpose cast on that gives an attractive and elastic edge and works in almost every type of project. Because of its structure, this cast on creates the first row of your knitting – which is usually considered the wrong side of the work for stockinette stitch.
As the name implies it starts with a long tail – which can be a challenge for a knitter to figure out how long to make the tail before you start making stitches on your needle. A general rule of thumb for this length is 3 times the length you need to form the number of stitches to cast on. Some knitters wrap the yarn around the needle the number of times that equals the number of stitches. I’ve used both methods and both work fairly well.
The cast on starts with a slip stitch (some knitters omit this step). You then form a “slingshot” by holding the needle in your right hand and using the thumb and forefinger of your left hand so that one of the two strands wraps around the thumb and the other wraps around the forefinger, forming a “Y”. The tail strand should go over the thumb and the working yarn should go over the forefinger.
To form a new stitch, put the needle under and into the loop on the thumb, then go over the yarn on the forefinger and bring the strand through the thumb loop. Drop the yarn off the thumb and gently tighten the loop on the needle. Repeat until you have the desired number of stitches.
There are several variations of this cast on. I have included a link for the Alternating Long Tail Cast on, which provides a way to alternate knit and purl stitches for a ribbing and gives a moderately elastic edge.