It's not exactly how I do it or learned how to do it. It's the same muscles you use when you swallow but I only tighten the muscles just above the larynx. It causes the larynx to move up some and that puts you in the head voice. It doesn't require extreme force and after you have done it a bit, you automatic tighten up that muscle group before you speak. If I want to return to the old voice, I have to try hard to relax the muscles.
One thing you can try is starting at your regular voice and gradually increase the pitch. At some point your voice will break to a higher pitch and that's the cross over point between the chest and head voice. Kind of a reverse puberty.