Just like any meditation technique, you'll want to find a nice quiet place. Preferably a darkened room. You can light a candle or two for added light. Nice relaxing music is welcome as well. However you normally meditate is what you should do to prepare.
I prefer to be lying down as I have a bad back, but you should get into whichever position you find the most comfortable. Not so comfortable that you'll easily fall asleep mind, just something where you won't soon find yourself shifting due to pain.
Breathe in deeply to the count of 4, hold for the count of 2 and exhale to the count of 4, hold to the count of 2 and repeat. Continue breathing deeply.
Allow your body to relax one muscle at a time. I usually start with my feet and work my way up to my calf muscles and my thighs. Then I relax my hips and stomach muscles, shoulders and arms, neck and face. Once you're relaxed begin to clear your mind. Feel yourself drifting in emptiness.
Imagine yourself under a waterfall. Feel the water pouring over your head, down your back, over your chest, down your arms, off your fingertips. Feel it flowing down your legs and over your feet.
Now feel the water go through you. Feel the water coursing through your body and leaving through the soles of your feet. Feel it in every cell of your body. Flowing freely.
Feel the water cleansing you as it flows through your body, taking with it any negative energy, flushing you through and leaving you clean and energized. Feel any negative energy leaving through your feet. Do this as long as you feel it's necessary.
Traditionally in Japan, the meditator would stand, sit or kneel under a real waterfall. This would be an exercise in mental strength as well as physical. If you want to try the traditional method but do not have access to an actual waterfall, you could try your shower on cold or cool. WARNING: If you choose to try the traditional method be aware of the risks. Hypothermia is very serious. If you feel yourself start to feel unwell or are shivering, stop the meditation and get warm.