All payments will be applied to any outstanding fees and accrued interest first. Any remainder will be applied towards the principal balance.
Scheduling an additional one-time payment on the same day you pay your full monthly payment is a great way to ensure most of your one-time payment is applied towards principal.
If you have Autopay scheduled, making an additional one-time payment on the same day as your full monthly autopayment is a great way to ensure most of your one-time payment is applied towards principal.