The tutorial shows how to build an amortization schedule in Excel to detail periodic payments on an amortizing loan or mortgage. Fill in the "Period" column.Fill in the column titled "Period" with your payment dates. If you prepay your mortgage you reduce the principal balance, reducing the interest due next month and every month forward. Does anyone know how to calculate the principal and interest payments after a one time extra principal payment on a mortgage in excel? Loan amortization schedule. 3. 4, Enter values, Loan summary. Enter your mortgage details here. Enter extra payment amount in relevant month rows. Want to pay off your mortgage faster?