Dmarc - Need help setting up DNS records so email passes dmarc

Mail sent by a third party sender I contract with is getting failed even though I have listed them in the DNS records. I can provide dmarc records and DNS records, give you access to the domain/site. I need you to help me so that the fails become passes. When the dmarc reports show this mail server passing, the project will be complete. You just need to give a short report on what you did, and what was wrong.