Lily bulbs can either be planted in the fall or the spring for summer blooms. First of all, you're going to dig a hole to plant your lily bulbs in. Lilies do best in well drained soil, so the bulbs don't rot. Typically when plant bulbs you want to plant them three times the size of the bulb. So if your lily bulb is 3 inches tall, you'll plant it 9 inches deep.
You also want the hole to be big enough to hold a bunch of lily bulbs. I would say at least 5 bulbs. They look better grown in clumps or mounds.