This works better if you're standing up. Lean your head all the way back, place your thumb at the corner of your jaw, and move it slightly to the center and down so you're pressing in the hollow between there and your windpipe, where the blood pumps the strongest.
Forgot to mention, your fingers should be touching the back of your neck. Your hand should be upside down. Like tipping a cup when you drink, tip your hand all the way back. Or, like this: