Do I need to clean the old gasket surface before installing these?

Yes. Scrape off all old gasket material and carbon buildup from the cylinder head and header flange. A clean, flat surface is how you keep these sealed tight. Wire brush or gasket scraper works fine.

What hardware do I use to bolt the headers back on?

You'll need header bolts — these gaskets don't include fasteners. Use the hardware that came with your headers, or source new header bolts if those are damaged. Torque them evenly in a star pattern to avoid warping.

Can I reuse the old gaskets if they still look okay?

No. Once you pull headers off, gaskets lose their seal. Reusing them almost guarantees exhaust leaks. Fresh gaskets are cheap insurance against a ticking, smelly header leak down the road.