While soil temperatures are still warm is the time to buy and plant bulbs. The safest time is now through October. I have planted bulbs as late as November, but you risk sparse root development that will affect flowering in future years. When selecting bulbs always buy the largest size bulb you can afford. Unless you are naturalizing with bulbs, bigger is better.
Larger bulbs produce bigger flowers that last longer.
If the bulb is not firm or even slightly soft, don't buy it. If it arrived in the mail soft, simply discard it. Planting bulbs is easy. Choose a sunny spot. The more sun the better.