Use the function wizard and HELP for details. That way, if you change your loan principal, the amortization table changes also. Remember that if you name cells, the reference becomes absolute automatically. Calculate the interest from the input data and divide the rate by the periods per year. Then multiply by the old balance.
New balance is old balance minus principal paid.
Copy down the line you just created except for the date . On the amortization table you submit, if you change payments per year, be sure you make the appropriate changes in payment dates. You may need to format the year cell as a number. Submit as indicated above, with your subtotal information in place.