The FAIR Principles call for good documentation and standardisation. You should already plan this in the design phase of your study.
An important aspect of the FAIR Principles is to make your data interoperable. Interoperability is the ability of data or tools from non-cooperating resources to integrate or work together with minimal effort. This means that your data should be readable for both machines and humans, and use terminologies, vocabularies or ontologies that are commonly used in your field.