Chop off the onion bottom with all the roots still intact. The more of a bottom you leave on, the better. Try for 1.5"-2" of attached "meat".
Allow to dry for a few hours to a couple days in a shaded, well-ventilated area to allow for callousing.Note: You might only need a few hours for callousing. If the cut portions are dry to the touch and slightly shriveled, it very well might be calloused enough.Note: I'm skipping the dry time for this Instructable.