In the United States, visit copyright.gov, pay a small fee (typically $45–$65), and upload your manuscript.
In India, go to copyright.gov.in, create an account, and file Form XIV online. You’ll pay a nominal fee and may need to submit a physical copy for verification.
Outside these countries, check your national copyright office.
Most nations follow the Berne Convention, so registration is not mandatory — but it provides enforceable legal proof of authorship.