In accordance with state law, all payments (including early and extra payments) are processed first to any interest that has accrued since the last payment. Earnest calculates interest at a simple daily rate, and you can find out more about that here. After the payment has been applied to the interest accrued, the remaining balance of the payment goes toward the principal balance of your loan.
Please note that any payments sent to Earnest by a third party on behalf of an Earnest client can only be applied as an extra payment. It is not possible to apply a third party payment towards the minimum monthly payment of a loan. For more information on this, click here.