I've done this often, but in the winter when we have little sun, it's a bit hard to keep them going and growing!  So I wait until spring and start a new batch.  This has been a bad year for it, however, because we've had very little sun.

http://dish.allrecipes.com/save-money-diy-fresh-green-onions/

Give it a try, it really is simple to do.  I just replanted mine into a medium sized flower pot full of our own garden soil from our regular outside garden.  It has aged manure and stuff already in it.