Waterfall meditation is somewhat different from both mirror meditation and candle meditation first because it is much more demanding and second because your focus is directed to your self and the relaxed rhythm of your breathing (rather than directed externally to a candle or a mirror). Waterfall meditation then is better suited for building levels of concentration than for quietening the conscious mind. However, if you are using a cold shower then as you become more adept at the exercise you can combine it with either candle or mirror meditation by placing the relevant object within sight.
Waterfall meditation involves simply standing, sitting or kneeling beneath a waterfall or shower and letting the cold water fall over you. What you will notice immediately is that the cold water causes your teeth to begin chattering and your breathing will become constricted as your chest involuntarily spasms and prevents air being breathed in and out smoothly. The test here is to be able to control your breathing so as to withstand the shock of the coldness of the water and continue to breathe normally. This requires focus and willpower and a mind clear of all other distractions.
To achieve this you need to keep your attention focused on breathing from your abdomen. If your consciousness is raised to your chest area you will find yourself spasming again. Breathing in this manner requires tremendous concentration and force of will. Given time and practice however the benefits of improved concentration and the ability to not be flustered by events happening around you will become ever more apparent. While always remembering to stay safe, you will be able to withstand cold water for increasing periods of time until your concentration is at such a level you are able to immediately relax your body, maintain your breathing pattern and barely notice the chill.
When your meditation has finished make sure you warm yourself and clench and unclench your fingers and toes, move your wrists and ankles in small circles, followed by rotating your elbows and knees too. This will bring your blood circulation back to normal. Remember, stay healthy and stay within your limits.