What screws do I use to attach these to existing wood posts?

Kristykay, Use pan‑head #10 × 1" exterior‑rated wood screws to attach the railing brackets to the wood posts. They should be stainless steel or coated to prevent corrosion, especially with pressure‑treated wood. The pan head fits the bracket holes properly and provides a secure, flush connection without splitting the post. - Your Deckorators Team