Direct sow in late spring for blooms in August and September. Plant in an area of the garden where Mammoth Sunflowers can grow to mammoth proportions. Sunflowers require direct sun, and tolerate dry conditions, but do need regular watering.
If growing for seed, allow the seeds to mature and dry on the plant (although you will loose some to birds and rodents). Harvest the mature seed heads and let them dry completely in a warm, dry place, safe from critters who can quickly consume the seeds. (Or, a generous gardener can leave the dry seed heads in the garden for the critters, or hang up the seed heads as bird feeders.)