If the plant has developed clusters in the ground you can divide these groupings in the winter or early spring, when the plants are not actively growing.
If you lift your bulbs for winter storage, in the spring you should remove the small side tubers that are attached to the main bulb.
Each of these divisions should have at least 2 visible buds or bumps.
The cut ends should be dusted with a fungicide powder.
Plant out into a well draining garden bed.
Plant with the 'eye' up and cover with about 1 inch of soil.