If you submitted Form 9465, Installment Agreement Request with your tax return, then the IRS will be sending you a notice in the mail approximately 30 days after the 7/15 deadline(usually longer especially this year).  The notice will indicate whether the Request was accepted or rejected.  Once the Request is accepted the method you requested to pay the installments will be activated.

Payment plans take weeks to approve so the payment will not happen until then ... they will send you a letter when they decide but for now you can expect the payment to happen anytime now ...  OR  you can make payments directly on the IRS web site now ...https://www.irs.gov/payments