Wash and trim the stems (use a sharp knife or scissors and cut a couple of centimetres at an angle) and then place in a clean vase of cool water. Add the flower food and make sure no leaves or foliage fall below the water line in the vase; remove these if they do. Your sunflowers will prefer cool temperatures and they won't very much like heat sources, drafts or direct sunlight. Removing individual spent leaves will help prolong the lifespan of your bouquet overall.