Face in cold water might/can do it....for some....but I would be more inclined to pour it over the back of my head and neck using a pitcher...over the tub or a sink. Kneeling, please...over the tub...just in case.

If you google or search YouTube for 'how to increase Vagus nerve tone', maybe include atrial fibrillation in the syntax, you'll find other examples of ways to increase Vagus nerve tone. One involves plucking at the tragus of your ears about 20 times using forefingers and thumbs.