On Loan Details, enter the amount, the annual interest rate, the term in months and the start date. The Schedule builds itself on a real reducing-balance basis: for each month it shows the opening balance, the payment (computed with a proper PMT formula), how much is interest, how much is principal and the closing balance, down to a zero at the end.
As you log payments made, the tracker flags anything DUE or OVERDUE, so a missed instalment cannot creep up on you. Yellow cells are your only inputs. The premium version adds a progress dashboard, a printable schedule and room for several loans at once. Opens in Excel or Google Sheets.