If you are mailing a check, you should either mail it with your tax return or mail it with a payment voucher. TurboTax will print a voucher for you or you can download one from the IRS: https://www.irs.gov/pub/irs-pdf/f1040v.pdf
As an alternative, you can use IRS Direct Pay and have the funds withdrawn from your bank account: https://www.irs.gov/payments/direct-pay
