An estimated tax payment for any SE income made between 9/1/2019 and 12/31/2019 is supposed to be paid-in to the IRS before 1/15/2020. (CA has the same deadline) .
But don't push the deadline, and keep a copy of any payment confirmations if you pay Online, or if you paper mail-in by check, use a tracking method at the Post Office as proof of date mailed, and receipt by each taxing authority.
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You can arrange an online 4th Qtr estimated payment by Federal Direct Debit here:
https://www.irs.gov/payments/direct-pay
Or use the 4th Qtr form on their PDF to mail it in:
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
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CA should have some similar website at the FTB webpage for either Online payment, or pay-by-mail PDF forms
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
