Quit watering them once the leaves start to turn yellow prior to harvesting if you can.....but we live in a rainy place, right?
To harvest, don't pull them up, dig them up gently with a fork. when the leaves start to turn yellow at the bottom....do a test on one or two plants to see if the bulbs are big enough. The trick is not to damage the outer skin covering.
After harvesting, try to avoid washing as much as possible. Dry the soil and gently rub off the remainder off, then hang them upside down in a cool dark place with good air circulation..... leave til all the leaves are dry. You can then cut off the leaves leaving a stalk about 4 inches long. Soft stemmed will keep longer (6 months) than hard stemmed (2-4 months).
Some people plat the leaves for storage. Enjoy.
I do it this way every year.