It's best if you wait until the stem and all turn brown. Then cut about 5 to 10 cm above ground, leaving a short stub in place until next spring. This natural process assures your bulb is mature will be well prepared for winter as well as full development toward putting on an even better show of more flowers next year.
You can, however, cut as soon as the leaves turn yellow and fall off, but understand that the bulb is not yet fully mature or hardened up for winter and early spring wet conditions (which lowers it's resistance to things like bulb rot).
