When to plantKeep bulbs cool and dry until you're ready to plant. You can plant when nights start getting cool. You can plant as long as the ground isn't frozen. I've even planted when the top inch of soil was frozen by frost. I just lifted the top slab of frozen earth, planted my bulbs, then replaced the slab.
DepthThe rule-of-thumb is to plant most bulbs at a depth 3x the height of the bulb. For larger Daffodils, this places the base of the bulb about 6-8 inches down. Smaller varieties, with smaller bulbs, should be planted at a shallower depth.
SpacingYou can space the bulbs about as far apart as the depth you plant them. It depends on the look you want. I like a natural, informal look, so I like to scatter the bulbs gently over the planting area, then plant them where they land.
Plant the bulbs with the nose, the pointy end, facing up, resting on the flat base.
TipsWhen planting larger areas, you can dig out the bed to the depth you want to plant, dig in any fertilizer, then place all the bulbs at once before back-filling with the soil you removed.
You can plant other, smaller bulbs at a shallower depth over the Daffodil bulbs before completely filling the hole. Bulbs that bloom earlier, such as Crocus (which are corms, not true bulbs, which serve the same function), will extend the Spring bloom season. Anything which blooms later may get smothered or covered by the Daffodil foliage, depending on how densely you've planted them.
Bulbs already have stored most of the food and energy they need to bloom once. To give them a boost, you can dig a little fertilizer into the bottom of each hole before placing the bulb. More important, to keep them blooming and spreading year after year, leave the leaves on the plants after they bloom until they turn brown. It will look messy, but that's how the bulbs store food for next year's blooms. Perennials and annuals can be planted alongside the bulbs; their leaves will help cover the dying foliage and keep the area looking neat.