When you convert part of your Citi credit card balance to an instalment plan, your first monthly instalment will be billed on the second statement after the plan has been set up. Your monthly repayment amount will then be included on your statement as part of the Minimum Payment Amount.
As with any credit card debt, you'll need to pay this minimum amount by the due date on your statement to keep your account in good standing.
If you want to pay off more than the minimum on your statement when you have a Fixed Payment Option, the additional money will automatically be applied to your regular account balance first. If your regular account balance is paid in full, any additional money you pay will go towards your instalment plan balance.