1. Login to the portal, click on your domain, and click the link "Manage DNSSEC"
2. If you have any existing DS records, you should delete them.
3. In the "Add a record" section, select "257-KSK" as a "Flag" and "13 - ECDSA Curve P-256 with SHA-256" as an algorithm.
4. In the "Publickey", enter the public key provided at the bottom of the "DNS" page for this domain in your CloudFlare account, and click Add.
If you see a red banner and stay on the same page, you entered inconsistent information. If you're brought back to the domain overview page, the operation succeeded.