As per above, for indoor bouquets, cut the main stem just before its flower bud has a chance to open to encourage side blooms.
For seeds,
Let the flower dry on or off the stem until the back of the head turns brown, the foliage turns yellow, the petals die down, and the seeds look plump and somewhat loose.
With sharp scissors or pruners, cut the head off the plant (about 6 inches below the flower head). Place in a container to catch loose seeds.
Lie the sunflower head on a flat, clean surface and grab a bowl to hold the seeds.
As per above, for indoor bouquets, cut the main stem just before its flower bud has a chance to open to encourage side blooms.
For seeds,