It is on Line 22 of the 2018 form 1040.
IF you print the 2018 1040 out, or look at the first page of a PDF document, both page one and page 2 of the new 1040 will print on one side of full sheet of paper.
Signature and date is in the middle of the page....if you end up printing and mail filing yourself. ....but if you are e-filing, then you can arrange a direct-debit from your bank account when you go thru the e-file process.
If you have already filed you have already filed, but did not arrange payment when you filed, you can also make that payment directly to the IRS on the IRS's DirectPay website. (IF you have to pay your state, you'll have to check your state's tax website for payment options...if not arranged when you filed) .
IRS DirectPay site
You need to make it clear as to whether you are asking about:
a) the final Payment you may OWE the IRS when finished preparing your tax return...
b) The tax payments and tax withholding that had already been done before filing...