After a light frost, dig them up and take them into a garage or basement (or other unheated building) to dry. I spread them on newspaper to minimize the mess. After a few days, the stems will have turned loose and you can easily pull them off the bulbs. Discard the stems (compost, etc.). Using a whisk broom, brush the loose dirt off the bulbs. Then store them in a single layer in a box (no cover) in a cool (not freezing) dry place till spring.