How do you massage a stiff neck by yourself?

There is no one answer to this question, because neck pain can have many different causes, and massage will only help with some, not all, of them. Even mild neck pain can be a sign of a medical condition, so it would be wise to see a well trained physical therapist, massage therapist, or chiropractor.

If the pain lasts more than a few days, is of unknown origin, or inhibits your activities of daily living, than it would be a good idea to have it checked by a doctor.

With that said, a common massage self-care exercise involves placing two tennis balls into an old sock and tying it shut, then laying on the floor with these under the neck, each ball on either side of the spine. Move slowly as you find the tender areas and press into them gently. A good rule of thumb is that if it feels good, it is good for you. If it feels bad, then you are pushing to hard or too fast. If you listen to your body it will tell you just how to do it. It works on the same principle as your pet cat or dog pushing back into you as you pet them, they don't need instructions, they just do what feels natural. Good luck with the neck pain.