We start by asking which records would be wrong to include. Columns are standardised to one definition each, and the original kept beside it. The handover is the point at which the criteria are verified, not assumed.
Source appraisal
Establishing where each field can come from, how current it is, and what it is permitted to be used for.
Format and encoding
Files prepared so they load first time, with the character handling settled up front.
Null discipline
Keeping empty and unknown distinct, so a filter can tell the difference.
We will say if the data does not support what you are planning.