Onions are grown from small bulbs. As untreated onion sets could flower prematurely, your best option would be to buy heat treated sets. Plant onions in mid-spring.
Worker soil down to a fine tilth. Add some organic compost from your kitchen composter or at least some fertilizer.
Drill holes in the soil that are only as deep as required for just the very top of the onion bulb to show through the soil. You want to space the onion bulbs at least 2 to 4 inches apart in rows that are 10 inches apart. If your soil is very light, you may not need to drill holes. You may just be able to push the onion bulb gently into the soil with the top showing. The reason you cannot do this when you have heavy soil is because the soil at the base of the onion set becomes compacted, which makes it hard for any roots to penetrate into it.
Water your onion sets immediately after planting them–especially in dry weather–and weed the area on a frequent basis for optimal conditions for growing vegetables. Heat treated onion sets may sprout a little slower than untreated sets, but they will catch up.
Regularly check your onion sets in train any dead leaves at the tip of the set, as birds can be a problem and dead leaves will make a handy handle for the birds to latch onto.