Since you want to e-file & mail a check to the IRS, choose to pay your taxes by check when you go through the e-file section, then transmit your return. Once your return is sent the program should print a payment voucher (1040-V) to mail in with your check.
The following is the 1040-V form in case it does not print out. The address to mail your payment will be on the form.
https://www.irs.gov/pub/irs-pdf/f1040v.pdf
