You can request an installment plan when you file the actual tax return. The IRS won't approve an installment plan if your return is on extension and not actually filed.
When you proceed to file your tax return (now you have until October 15th), you are asked how you want to pay your taxes. You would choose "Installment Plan" and you'll be led to complete the required form.
Remember that an extension extends your time to file, not to pay. If your return is ready to go, but you just don't have the money, file ASAP and get that installment plan started.
https://ttlc.intuit.com/questions/2840416-can-i-set-up-a-payment-plan-for-my-taxes
