To do this stretch the stretcher lies face up. The partner stands at the head of the stretcher and places their left hand on the stretchers shoulder and thier right hand under their head, helping them to flex thier head and neck so their chin is touching their chest. From this position rotate the head 45 degrees to the right to fully lengthen the muscle, this is your staring point. From this position direct the stretcher to slowly push their head and neck back towards the corner of the table, remembering to breathe normally, as the partner provides matching resistance. After this the stretcher relaxes and inhales deeply. On the exhale the stretcher should try to flex their head and neck to bring their chin closer to their chest.
Information obtained from McAtee, R. & Charland, J. (1993) Facilitated Stretching
Images from http://www.rad.washington.edu/academics/academic-sections/msk/muscle-atlas/upper-body/levator-scapulae