On 13th July 2020 the Cabinet Office published The Border Operating Model: a guide to how the border with the European Union will work after the transition period.
This document details that on 1st January 2021 the transition period with the European Union will end, and the United Kingdom will operate a full, external border as a sovereign nation. This means that controls will be placed on the movement of goods between Great Britain and the EU.
The UK Government has taken the decision to introduce the new border controls in three stages up until 1st July 2021.
The Border Operating Model can be found here https://www.gov.uk/government/publications/the-border-operating-model
Step-by-step guides have also been published for Importing and Exporting.