Select a location with full sun where your onions won't be shaded by other plants.
Soil needs to be well-drained, loose, and rich in nitrogen; compact soil affects bulb development. Till in aged manure or fertilizer the fall before planting. Onions are heavy feeders and need constant nourishment to produce big bulbs. At planting time, you can mix in some nitrogen fertilizer, too, and side dress every few weeks until the bulbing process begins. Read more at http://www.almanac.com/plant/onions
For any of the various onions, it is time to harvest the onions when the bulbs begin to pop up from the soil. To prevent any breaking of the bulbs, dig the onions up from the soil with a shovel carefully removing the bulbs. After harvest, dry the onions with the green tops in a cool dry area until the outter layer becomes dry and flakey. Then cut the the tops off for later storage.