Need help trimming roots on house plant

Hello,
I just bought a spath plant to put on our window sill. I know very little about plants or gardening (go figure...I live and grew up in the heart of New York City)
The roots of this plant are at the edge of the pot i bought it in but rather than replant it in a bigger pot i would like to keep it the same size it is now. The clerk at the store told me I can trim the roots but ive never done it before and i dont want to kill the plant.
Can anyone with experience offer some suggestions or advice on how to do this ? I cant find a good website with clear instructions for doing this.
Thanks for any help !
-A.P.
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I simply use a serrated edge knife and saw off about an inch or so from the bottom, then slice it the same way on four sides. More or less as the size of the rootball dictates- I wouldn't want to remove more than 40% (or so) all at once unless you want to reduce the foliage similarly.
The only thing you need to do afterwards is water it well and keep it in very reduced lighting for a couple of days. Mist if you have low humidity, and make sure you have ample drainage.
Plants are fairly resilient- it will be fine.
-- Toni South Florida USA Zone 10b http://ww.cearbhaill.com
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