When you access the Cloudflare CDN page, you will see a list of all domain names related to your account. You will see a ‘manage’ option in front of all the domain names in which you have enabled Cloudflare. This option will provide you with the access to all the management options for the selected domain.
Once you click on ‘manage’, on the first tab you will see all the sub-domains present on your web hosting account and information on whether the Cloudflare CDN is activate for them or not. In order to activate the Cloudflare CDN service for any of the sub-domains, all you have to do is click on the ‘Activate’ button.