What happens next?

How to harvest sunflower seeds
Wait until the sunflower begins to wilt.
Sunflowers are ready to harvest once the heads become brown. If you have a particularly wet season however, they can go mouldy [if this is the case you will need to cut off the head once the back of it is yellowing and allow the drying process to continue in a greenhouse or shed].
You should prepare them for the drying process once the back of the head begins to turn yellow to yellow-brown. In order to harvest the seeds, the sunflower head must be completely dry.