HMRC have released a letter to VAT registered businesses that trade with the EU and / or the rest of the world. This letter outlines all the actions that need to be taken to continue trading from 1st January 2021 when the transition period ends.
As noted on the HMRC website "These actions will not change regardless of the outcome of the government’s negotiations with the EU."
So, if you haven't received your letter yet, you can download it from the HMRC website here.
If you have any queries, do get in touch.