@kp79
Since I've been making 4 quarterly estimated tax payments for ages I have opted to use the Electronic Federal Tax Payment System. It's pretty easy to set up a string of future payments and if you decide you need to change a particular payment or payments, it's also pretty easy to modify the payment or payments.
The disadvantage is that you loose the "float" but in these days of near zero interest rates that amounts to pennies.
If you want to mail in your payments you can simply print out an empty 1040-ES if you're using a desktop product - not sure about the online products. Or you can download the Form 1040-ES booklet and use the empty vouchers at the bottom of the booklet. https://www.irs.gov/forms-pubs/about-form-1040-es
