If you go to a good supermarket, look over their roots you will see eyes. Get one with as many eyes as you can. At home wash with soap and warm water, rinse Cut it so each peace has an eye. Plant it. It will take about a year. Remember frost will kill it.
My mother used to buy a good, fresh root at the market -- one with plenty of eyes, and keep it under the sink in a bucket of damp sand. If the weather was warm enough, she would cut it into pieces, with one shoot per piece, and plant it all outside. Small (6-10") young shoots dipped in miso paste are old-fashioned beer munchies in Japan.
The supermarket, or better still an Asian market, is your best bet. Occasionally stores will spray roots with a growth inhibitor, so the roots won't sprout, but that's a chance you take, and it's becoming a less popular practice these days. Buy the freshest pieces you can, nice and firm and not too gnarled and wrinkly.
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