I have recorded a video: here
Explanations bellow.
Hold the needle with the stitches on it in your left hand. Loop the yarn that goes from the bottom of the first stitch to ball over the top of the forefinger of your right hand and hold the empty needle in your right hand.
Move your left hand so that both needles are held in your left hand between your thumb and your forefinger. Using your right forefinger, wrap the yarn from the stitch on the left needle up and over the top of the right needle in a clockwise motion.
Slide the right needle (with the loop of yarn on it) down and towards you through the center of the stitch that is on the left needle.
Pull the yarn that is around the tip of the right needle down and through the stitch on the left needle to create a new loop ('stitch') on the right needle.
Slip the new stitch and the stitch from the left needle to which it is connected completely off the tip of the left needle.