I have about 2 dozen giant sunflower heads that got killed in the freeze last night.  I think they were about ready anyway.  Now what do I do to harvest the seeds?

Cut the heads off leaving a few inches (3-6) of stalk.
Hanging them to dry is best. You want them to dry out fairly quickly or they may rot on the backside. When they are dry enough that the seeds will pop out by rubbing your hand across the head, remove them by using your hand as a scrub brush.

A glove might help.

When I grew them, the heads started rotting on the backside after a week or so after I hung them in the garage. The heads were not "dried stiff" when I deseeded them. The heads were huge, the seeds were meh. The birds loved them.