"...what form do I use (I can't just stick a check in an envelope and mail it to the IRS)?"
The easiest and safest way to make an estimated tax payment is online, via the IRS website:
https://www.irs.gov/payments
If you do this, be sure to indicate the tax year for which you are making the payment.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
