Sunflowers grow best in full sun, getting 6+ hours per day. They will grow in just about any type of soil, but because they have deep roots and are heavy feeders, you may want to amend the soil before sowing. Till the soil 12"-18" deep and amend it with compost, seaweed, fish emulsion, or composted manure. Also, consider a few side-dressings or supplemental feedings during the growing season. Sow seeds directly outdoors, because transplanted seedlings tend to grow less vigorously.
Also, seed generously to make up for seeds or seedlings that may be lost to birds, squirrels, or slugs. Then, for the largest sunflowers, when your plants are 2"-3" tall, thin them to two feet apart. When plants are closer together, they compete for nutrients and don't grow as large. Keep the planting area weed-free to eliminate nutrient and moisture competition.
Sunflowers are drought tolerant once established, but will grow larger with even moisture. Stake plants in windy conditions or if they become top-heavy. To keep birds or squirrels from munching on the seed heads, you can cover them with tulle, a fine netting, or burlap.
Check out our Mammoth and Mammoth Russian Sunflowers. They are open-pollinated varieties that can reach 12' tall, with impressively large, 12" wide flower heads.