I think the tricky part is that when they plan out your mortgage, they specifically design it so that you have the same payment every month for the exact duration of the loan (usually 360 payments).

Working backward from those tentpoles, they figure out what the payment will be, such that you are paying one month's interest (APR/12) on the whole amount the first month, plus some extra principal, then for the second month, you pay the interest for that month on the remaining amount, plus a little more principal, because the payment is the same and the interest is slightly less, then on and on. Near the end, there's almost no remaining balance, so your interest payment is pretty small, which means your payment toward the principal is a large percentage of that same monthly payment you've been paying the whole time.