Centaurea cyanus is an annual that gets about 3' tall, and blooms come in blue, white, pink, red, and purple. It is also commonly refered to as Cornflower, and Butonniere flower. When Bachelor's Button flowers fade they don't leave behind an obvious seed pod, but rather a seed head. If you don't know how to collect Bachelor's Button seeds you may think that the flower wasn't pollinated. But if you pluck the flower's seed head pictured above you'll get a few seeds for your collection.
Take the seed head in your fingers and gently break it apart by rolling it in the palm of your hand, or between your fingers, to reveal the seeds inside. Bachelor's Button seeds look like little bullets (or badminton birdies) with a tuft of hair at the blunt end. These are the seed you're looking for and want to save. Once you've collected all the seeds you need you can set them on a paper plate to dry for a couple of days and then store the seeds in a paper envelope or plastic seed baggie.
How to Collect Bachelor's Button/Cornflower Seeds Video
Bachelor's Buttons is incredibly easy to grow in gardens in a variety of soils-they're downright weedy most of the time, but not too aggressive in my garden. My biggest problem with growing this annual in my garden is that I forget what the foliage looks like in the spring and I end up weeding the plants.