If your payment plan has not yet been approved, send in a payment before the due date of the return with the voucher. It can take up to 30 days for the IRS to accept your plan, and the IRS will not honor the agreement if the 1st installment is not received by 4/18/17.
You can also pay on the IRS website directly. https://www.irs.gov/payments
Note: Your tax return may be accepted in 24-48 hours. This not the same as having your installment plan accepted.