Generate a full month-by-month or year-by-year loan payment schedule
In the early months, your loan balance is highest, so the interest charge (balance × monthly rate) is highest. As you pay down principal, less of each payment goes to interest and more goes to principal. This is why the last few payments are almost entirely principal.
Any extra amount you pay goes directly to reducing the principal balance. This reduces the interest charged in every future month, compounding the savings. Even $100/month extra on a 30-year mortgage can save years of payments and thousands in interest.