It can take several months before IRS will approve or deny the installment agreement application. If you'd like to make payments toward the balance before you get the letter approving or denying the application (the letter will inform you when the automatic payments will begin), you can do so electronically through the IRS Direct Pay website:
https://www.irs.gov/Payments/Direct-Pay
Or you can send payments in via check through the mail, you should always accompany the check with a payment voucher to identify the payment for the correct tax year. You can download and fill in a 1040V payment voucher direct from the IRS website.
https://www.irs.gov/pub/irs-pdf/f1040v.pdf