Hi i find the easiest way is to buy onion sets,there is loads of different types it depends what you want,don't buy any now they will be past it they should have gone in a few months ago,i just prep the soil and rake it over to level off then make a small hole with a dibbler about 6 to 8" apart and push them in so the tops are just showing and water in, keep checking them for a few weeks until they take root the birds tend to pull them out, if this happens just push it back in and water.I grow a few hundred to last all year,cooking onions and red onions,they will keep well into the following year if you store them right,i string them up in the shed so they are not touching, keep an eye on them if one or two don't look too good throw them out.