Login to your Cloudflare Account, and go to DNS.
Select Add record.
Select TXT as the Type and fill out the fields with the following info:
Host: Enter the host name for the TXT record. For example, enter @ for an SPF record, or enter the value _dmarc for a DMARC record.
TXT Value: The value you are setting as the destination of the Host.
TTL: Determine how long the server should cache information.
When finished entering information, select Save.
Create a free OnDMARC account to test your configuration.