I am assuming you are amending a 2019 income tax return as we are still in 2020. You would include the Form 1099-G in front of your 1040X Form. You would also attach any other tax forms that changed as a result the 1099-G behind your Form 1040X.
Please see the following for additional instructions.
Please see the following link for a few more tips on amended returns.
Tips for filing an amended return