Howdy Chris. I've grown a lot onions for a lot of years here in Texas and maybe learned a few thing along the way. My conditions are very different here from what you grow in, but take that into account.
First comes the choice of the set or seed if you prefer. You will need a long day variety in your area. Select recommended varieties for your specific area.
Sets should be planted as shallow as possible. The soil should be prepared well in advance of planting. Additives like bone meal, cow manure, and ground rock phosphorous should be worked into the soil well in advance of planting. Also, they need a relatively neutral ph so add lime if needed again well in advance of planting. Soil preparation is extremely important. I constantly try to improve my soils with cover crops and additives. The big onions are "grown" well before you actually plant, if you understand what I mean.
Now for planting...Give them plenty of room to grow in...4-6 inch diameter onions need space. You want to encourage as many leafs as possible, as they make the big bulbs. As far as fertilizer goes after planting, go with high nitrogen. Your "P" and "K" should have already been worked into the soil before planting. I like fish emulsion applied weekly during growing season.
I'm a complete believer in loosening the dirt around the bulb or better yet pulling dirt back some. Do this when the bulb really takes off in growth. They need regular water about 1 inch a week. Do not under any circumstances let them get extremely dry during growth. Withdraw water about a week before harvest to improve storage.
Those are the important big ticket items that come to mind. I'm sure I've forgot something so a question or two or three might jog my memory and would be welcome.
I love to grow big onions....last year's crop was topped by a 4 plus pounder, delicious. Very fun and very rewarding.