I would typically go  to the prior year return and input that return, as if I were going to prepare it. In this case reproduce the 2020 tax return. This gives you an opportunity to review the work of the prior preparer and, if necessary, be ready to go for an amended return. Then do the proforma to 2021 and print an organizer to give the client, if they wish.

If there is an REP charge for the prior year, make the SSN all 9's and there will be no charge. When you go to 2021 remember to change SSN back to correct one.