If this is your first try at growing and harvesting sunflower seeds, you'll probably find it hard to get all of the seeds out of the flowers. Some will fall out naturally, while the others seem to be stuck inside the flower head culls (or seed holders). The secret to easy sunflower seed removal is in the drying process.
Leave the sunflower heads attached to the plant stalk, until the petals have dropped off and the remainder of the flower head has dried and turned brown. Don't be tempted to remove the flower heads before this occurs, even if it seems the birds are having a feast, feeding on the seeds.
Cut the flower stalk down about 2 feet from the flower, using a sharp knife or garden clippers.
Place a paper bag over each flower head, after you have cut them from the stalk. Scrunch up the bag at the top and tie a string or use a rubber band around the bag top to keep it attached to the flower stalk. This will catch falling seeds.
Hang the sunflower heads upside down, from the stalks. Tie a string or fine cord to the ends of the stalks and attach the other end of the string to garage or garden shed rafters with hooks or nails. It should be a well-ventilated, dry location. It will take several weeks for them to dry out.
Remove the dried sunflowers from the hanging location when they are completely dry and the seeds easily fall off when you rub your hands over the flower head. Check the dryness of the flower head by opening the bag and running your fingers across the flower head. If they are not ready yet, retie the bag onto the flower stalk.
Cut the string from the hanging stalks of sunflower heads. Untie the paper bags. Before removing the flower head from the bag, rub your hand over it, allowing the seeds to fall into the paper bag. If you keep a compost pile, throw the spent sunflowers into it.
Sort through the seeds. Place any underdeveloped or bad seeds out for the birds to eat. Store the sunflower seeds in a closed container or process them by roasting, as you prefer. If you have used heirloom (original, not hybrid) seeds, save a few to plant next year.