In the first week, choose a small set of critical data elements and document where they come from. Speak with the people who create, transform, and use them.
During the next two weeks, add lightweight checks for freshness, completeness, uniqueness, and accepted values. Publish the results where owners can see them, and agree on what requires action.
Use the final week to prioritize fixes by decision impact. A quality program becomes sustainable when ownership and follow-up are part of normal work rather than a separate cleanup campaign.