Onion sets are small, immature bulbs about 1/2 to 1 1/8 inches in diameter--these are grown from thickly planted seeds the previous season. Plant sets about 1 inch deep--just enough to cover the small bulbs. The closer you plant onions together the smaller the bulb size at harvest--so you can thin them to 1 1/2 for smaller onions to 4 inches apart for larger onions.

Here is a link that might be useful: HarvestToTable.com

Thanks!

If your planting them for bulb onions, plant as stated above, if you're planting them for green onions, plant them deeper depending upon how much white vs green you want. I find I do better growing bulb onions from slips of the varieties that do well in my area and I only use the sets for green onions.