You can pretty much set up the amount you want to pay in your installment plan, and it can be changed after it is set up, but keep in mind, the more you pay now (and the sooner you pay in full) the less you will have to pay in interest fees and penalties. This link can provide more information:
https://turbotax.intuit.com/tax-tips/tax-payments/facts-about-irs-payment-plans/L1zHPFVDI
