If you know the term of a loan and the total periodic payment, an easy way to calculate an amortization schedule is to do the following: Starting in month one, multiply the loan balance by the periodic interest rate. This will be the interest amount of the first month's payment. Subtract that amount from the total payment, which will give you the principal amount.
To calculate the next months' interest and principal payments, subtract the principal payment made in month one from the loan balance, and then repeat the steps from above.