You actually have several choices/ways to pay the SE tax. You can mail a check if you like. Just get the 1040-ES forms for 2019 directly from the IRS at https://www.irs.gov/pub/irs-pdf/f1040es.pdf. The forms needed for each quarter are at the end of that document. Pay attention to the due days for each quarter, because those dates are not exactly by the quarter. That document also contains the address to mail your check and form 1040-ES to, based on your location.
The method I use is to just pay each quarter online directly from my business bank account. You can pay online by credit card each quarter too. Just go to http://www.irs.gov/payments and make your selections from there. If you pay online, make sure you print your receipt not only to file with your business records, but as proof if you're ever audited on it.
When you make a payment online, it's important to insure you have the funds in the account to pay them with. Otherwise, not only will you incur NSF charges for the failed attempt by your bank, you'll also pay a fee to the IRS, and those fees are just flat out not deductible.
