Sunflower seeds are ready to be taken from the flower when the back of the sunflower's head turns brown. Most of the leaves will have fallen, and the seeds will have plumped and developed black and white stripes.
Cut the head off of the sunflower but retain a foot or two of stem. Rub the head by hand to remove the seeds.
Tie a paper bag around the head of the sunflower after you cut it from the stem. Hang the head upside down in a warm area. The falling seeds will be gathered in the bag. Check the bag every few days to see how many seeds have fallen.
According to sunflowerguide.com, a paper bag will help save a naturally drying plant from animals that want the seeds too. Just before harvest time, tie a small brown paper bag over the head of the plant. The site says the plant will still "breathe" this way but the seeds won't get moldy.