Hi,
You need to wait. If you cut off the flowers, the seeds have not developed all the way. If the flowers have died off and the seeds are almost ready to go, you might be successful. It sounds though as if you want to cut fresh blooms which will not work.
Remy

Wait to harvest until they are completely dried up, as you would with most any seed. Be careful when cleaning them, as the seedheads are sharp and can cause infection if you get pricked. The seeds are the light-colored 'space shuttles'.

What I would like to know is, will they re-seed if I scatter their seeds over a bare area?
Paul

Paul,
The answer is maybe, lol. I know, not the answer you wanted to hear. With re-seeders, some people will tell you, "oh it re-seeds everywhere. It is so easy to grow, like a weed." Then you will get people who say, "I planted that and have never seen a seedling." Cornflower seed is so cheap though, it is a good one to try to get to fill an area. If the area of ground is very bare, I would try to loosen the soil up a bit before scattering the seed so they have spots to take hold. Or if possible, plant a few plants. Sometimes people have better luck with plants re-seeding for them.
Remy