Using DNS-O-Matic allows you to pick and choose what dynamic DNS services you want to notify, all from one easy-to-use interface.
Below are the settings for both DNS-O-Matic and Cloudflare. You'll want to make sure to use your own information in place of anything wrapped with < >.
Username: <CLOUDFLARE ACCOUNT EMAIL>
API Key: <CLOUDFLARE CLIENT API KEY>
Cloudflare DNS Settings (Cloudflare.com > DNS > DNS Records)
|Type: A||Type: CNAME||Type: CNAME|
|Name: dynamic||Name: <example.com>||Name: www|
|Value: <WAN IP from DNS-O-Matic>||Value: dynamic.<example.com>||Value: <example.com>|
Your Cloudflare API Key is found in the My Profile section, linked from the top right of the Cloudflare dashboard.
One limit: if you have multiple A records for the hostname you are updating, DNS-O-Matic will update the first record found, not others. (For dynamic DNS, multiple A records for a single hostname is unlikely.) One possible workaround would be to change additional records to CNAME records pointing to the original A record as done in the example records above.