Put the current balance in this box.
For example, if the current balance is $253.28, put 253.28 without the dollar sign in the box.

Next, put your monthly payment in this box.
For example, if the monthly payment is $25, put 25 without the dollar sign in the box.

Next, put in this box the annual interest rate.
If the interest rate is 5.25% put 5.25 in the box without the % sign.

Finally, put in this box the number of payments.
For example, if you want to see what the balance will be after you've made 6 more payments, put 6 in the box.

And click this button

You can do this again with different numbers. Just hit the clear button.