Change the nameservers at your registrar

Cloudflare shows two nameservers (names ending in .ns.cloudflare.com). Log in at your registrar, find Nameservers (or DNS settings), replace the existing ones with Cloudflare's two, save. Back in Cloudflare tap Continue.

If your registrar has DNSSEC turned on, turn it off first, or the domain goes dark. The change can take hours; Cloudflare e-mails you when the domain is Active.