After starting with DMARC Analyzer most customers find out that their SPF is not (fully) implemented. Based on the DMARC reports you’ll learn which servers should be authorised in and added to your SPF records.
Also see our full article about SPF.
In order to create SPF records, the web offers pretty good information. We recommend to consult the following websites which have nice SPF record generators.
Configure SPF records to work with G Suite