Since our sunflowers dried naturally, we didn't go to much trouble, but if you're concerned about animals getting to your seeds, you want more detailed instruction on when to harvest, or you want more of a method to the process, here's a step-by-step you may want to reference: "How to Harvest Sunflower Seed" on WikiHow.
My son sorted and the seeds out, removing "questionable" looking ones and any debris from the flower head that had gotten mixed in. We put seeds (about 1.5 cups) into a container, added enough water to have covered them if they didn't float, and stirred in 1/4 cup of salt. I allowed them to soak between 8-10 hours.
Preheated the oven to 300 degrees F; drained off the water from the seeds, dumped the wet seeds onto a paper towel and jostled them around a bit, then placed them in a single layer (as much as possible) on a large baking sheet. Baked for 30-40 minutes. I was waiting for them to crack open, since one instructional I read stated they would. After 50 minutes (oops), they still hadn't cracked and I noticed a few of them starting to brown slightly so I pulled them out. They were a bit more dry that I would have liked, but still turned out way better than I expected.30-40 minutes is the right cook time, don't wait for browning or splitting open!
It was a great activity following our study on flowers and seeds for homeschool science. Here's another WikiHow article with different methods of roasting seeds including roasting with shells removed, boiling seeds rather than soaking overnight, and some interesting seasoning suggestions at the end.