Log in to your Google Domains account, select your 'Domain', and go to the 'DNS' section.
Scroll down to the 'Custom resource records' and create the following record:
Name: Enter the host name for the TXT record. For example, enter @ for an SPF record, or enter the value _dmarc for a DMARC record.
Data: The value you are setting as the destination of the Host.
TTL: Determine how long the server should cache information.
Click 'Add' to publish the new record.
Manage Resource Records with Google Domains
Create a free OnDMARC account to test your configuration.