Onions are usually planted as 'sets' (small onions) in rows 15 cm apart and 15 cm between 'sets' or maybe even more space if possible. This allows more food and water for each bulb and makes hoeing between them easier. The sets are available in early spring. Insert only the bottom half or use a handful of sand to keep the them upright. Birds tend to pull them up so keep checking and re-plant any that have been disturbed. Alternatively they can be brought on in modules so that they have developed a root system before planting out.
Japanese varieties available in late autumn, are planted in October/November and they are ready to harvest earlier the following summer.
When the bulbs are mature the tops turn yellow and die back. They must be allowed to shrivel completely so that there is no moisture left in the old topgrowth. Bending the tops over used to be the standard advice to encourage the bulbs to ripen, but this damages the tissues and allows rotting organisms to enter, so it should be avoided. Some growers ease the bulbs up slightly with a fork to encourage this dieback by breaking the roots. After lifting leave them on wire mesh and covered to keep the rain off, but with a good flow of air around them; in a few weeks they can be strung up, placed in net bags (the ones oranges come in are useful) or stacked in mesh trays, in a cool dry place for storage. Only keep bulbs with completely dry tops, any with thick necks will retain some moisture and will rot in storage - use these right away.
Onion Fly (Delia antiqua) - the maggots eat into the bulbs before migrating to the soil where they pupate. Lift and destroy affected plants to prevent pupation.
Thrips - nibble the foliage leaving small spots of dead tissue. Use garlic spray.
Downy Mildew can attack if conditions are present, but planting with good spacing to allow air to circulate should prevent it.
Onion White Rot is the most serious disease and must be avoided if at all possible. Raising from seed should prevent it being brought into the garden.