The directions mean do not staple the payment to the amended tax return, Form 1040-X. You can pay the taxes owed on the amended return with a check mailed in the same envelope as the tax return.
Or -
You can also pay the taxes owed on the IRS payment website using Direct Pay. On the website you would select Amended Return for the Reason. Then select 1040-X for Apply Payment To. Then select 2019 for Tax Period for Payment - https://www.irs.gov/payments
