Government guidance was published on 31 August 2021 on "Operating a Freeport customs site". The conditions relate to site security (e.g. making sure that the site is enclosed with designated entrances and exits), record-keeping (e.g. compliance with record-keeping requirements, providing information to officials, and providing facilities on site for inspection), and rule-following (e.g. preventing banned activities, following health and safety requirements, refusing prohibited goods, allowing examination of goods and preventing movement of goods out of the site otherwise than in accordance with the legislation).
The operator will be responsible for putting robust policies and procedures in place to protect its business and the supply chain from risks. This will be key. It will also be the responsibility of the operator to ensure that any business carrying out activity within the site has told HMRC of its intention to do so, and has any necessary authorisation.
More guidance is promised but in the meantime, the list of requirements published so far can be found here: https://www.gov.uk/guidance/operating-a-freeport-customs-site
If you want to operate a Freeport customs site, you’ll need to show HMRC that you can comply with the conditions of Freeport customs site designation and safety and security standards.