The following recommended resources can be of assistance when you need to:
Metadata is data about data - the information needed to make you dataset discoverable, citable, and usable by others.
Metadata is available from a variety of sources: data repository records, codebooks, data dictionaries, user guides, methodology guides, README files, etc. This documentation provides users with the information they need to understand, interpret, and repurpose your data.
Remember that it is best to document your data throughout your research project, rather than at the end.