As of 1 January 2021 the United Kingdom is considered as a third country in terms of customs regulations. Special rules by the Withdrawal Agreement are applied to Northerm-Ireland.
Customs formalities (customs declaration, safety and security data) and trade policy measures (prohibitions and restrictions) are applied at the EU customs border. Some goods (e.g. waste, dangerous chemicals) need import or export licenses. In addition to value added tax, if preferential origin is not proved, customs duty shall be paid during the customs procedure of release for free circulation.
The guidance note by the European Commision provides assistance to step-by-step preparation.
Checklist by the UK Government for EU-based businesses trading with the UK is available at:
The websites of the Netherlands and France provide all stakeholders with useful information regarding the procedure at the ports.
https://www.getreadyforbrexit.eu/en/
https://www.douane.gouv.fr/dossier/working-through-brexit-together