Every plan includes SSL (HTTPS) at no extra cost, and your free .yclas.com address already uses it.
When you connect your own domain (My sites > your site > Custom domain), you add one CNAME record at your domain registrar. We then issue the certificate and switch HTTPS on automatically once the record is in place. You don't need a Cloudflare account or a certificate of your own.
Sites that connected their domain earlier through their own Cloudflare account keep working as they are. Open a support ticket if you'd like help moving to the new setup.