I filed for an extension and didnt make an estimated payment, but now i want to make an estimated payment for my 2018 federal taxes. How do i do this on turbotax?

You can pay on the IRS site.

https://www.irs.gov/payments

When you requested your extension you were supposed to pay your estimated tax due.  Did you enter the payment you made into your tax return?  Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes  

When you open your IRS  account you will choose to make a 2018 form 1040 payment ... there is no longer a 2018 extension or estimated option any longer.

Or mail in a check with a 1040-V    <a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/f1040v.pdf">https://www.irs.gov/pub/irs-pdf/f1040v.pdf</a>

also be sure to enter this payment i TT before submitting return .  TT will not pick up the payment automatically.