When you buy bulbs you are, in effect, buying packaged plants. Stem, leaves, and flower bud, the complete package is all wrapped up in the fleshy part of the bulb. This acts first as a protective shield and then as a source of food when growth starts in earnest in spring.
For this reason, bigger is always better when it comes to selecting bulbs. Also avoid damaged bulbs, and those with soft spots or signs of mold.
Generally speaking, if you can choose good onions, you will have no difficulty choosing good bulbs.
Spring flowering bulbs can be planted as soon as soil temperatures drop below 60 degrees F. In the US the best planting times occur from late September through November and even into December in some of the more southerly regions.
After planting, bulbs quickly form roots and send up a short stem. These remain submerged in and protected by the soil throughout winter until spring temperatures trigger renewed growth. As a rule of thumb, larger bulbs (tulips, daffodils, hyacinths, and the like) should be planted in a hole 8 inches deep; smaller flowers (grape hyacinths, crocus, etc.) 4 to 5 inches deep.
Avoid single-line planting unless you want a stiff formal effect. In contrast, bulbs planted to form swirls or drifts of color in the spring have greatest appeal. Some gardeners take a handful of bulbs and cast them on the ground, and plant them where they land for an informal effect. Plant short varieties in front, taller ones in back.
In larger beds, it's simplest to remove all the soil to a depth of about 8 inches. Loosen the soil below and amend it with sand or peat if poor drainage might be a problem.
Set out the bulbs, pointed side up, and cover with the loosened soil that has been enriched with compost, aged manure, or a sprinkling of 10-10-10 or similar fertilizer. Add a tablespoon of super phosphate or Bulb Booster rather than bone meal, as the latter can attract rodents.
Dig individual holes when planting in an established bed. In these instances add a handful of sifted compost and peat moss (mixed 50-50) into the bottom of the hole, place in the bulb, and cover it with a second handful of the same mix, add fertilizer before covering the hole. Water well at this stage, after which normal snow and rainfall will provide enough moisture for the bulb's needs.