How do you copyright game ideas? Say one has a game idea, pretty worked out. What would be next best steps to take, without being copied/stolen in the process?

How do you copyright game ideas? Say one has a game idea, pretty worked out. What would be next best steps to take, without being copied/stolen in the process? Unlike startups, with boardgames ideas matter much more than execution. Copyright is the only protection for successful games from being duplicated, since materials are so cheap.