Can you use a spreadsheet?

If so, label the columns like this:

date, principle, interest, payment

The principle column is the running total of how much you owe, starting with how much you borrowed originally. Each month after that it is the previous month's principal balance, plus the interest for that month, minus the payment.

The interest column is the principle amount times the interest rate (as a decimal, i.e. 5% interest is 0.05), divided by 12 (months in a year).

If the payment is the same every month, you will have 60 rows for a 60-month loan. If you make bigger payments, you will have fewer rows (because you've paid the loan off sooner!)

Sum the numbers in the "interest" column to see how much total interest you will pay with different payment amounts. Making bigger payments has a greater effect early in the loan.

You can do this on paper too, but it takes longer.