The metadata schema should conform to the open-source STAC standard, which is available here: https://github.com/radiantearth/stac-spec/blob/master/collection-spec/json-schema/collection.json
Additional descriptions for each field within the STAC standard, are available here: https://github.com/radiantearth/stac-spec/blob/master/collection-spec/collection-spec.md
The resulting metadata should then be provided in a .json file.
An example metadata file is available for download at the bottom of this article:
(please note this file includes additional comments which need to be removed to be valid JSON)