Long ago, we paid off our 10% mortgage early. We paid a few bucks to the loan holder (it was a lot simpler back then) to get a printout of our mortgage payments to go. It was a line by line list of each payment to come, showing what $ for Principal, and what $ for Interest.
So each month, we would send in the current principal and interest amount, plus an integral number of months of Principal payments all added up.
So we could then cross off the current principal amount, and advance X number of months into the future for next month's principal payment. By making only integral month(s) of extra Principal payment, it kept it very easy to keep track. And easy to see when into the future that the value of X would need to be reduced as Principal payment line values grew higher, and determine when to drop a month off of the value of X.
In the early part of the loan, an extra month's Principal was very cheap! And the opposite as the end of the mortgage approaches. I think we paid it off in 8 or 9 years, we did not know about prepayment until we were a couple years into it, at least.
In retrospect, a 15 year loan would have been another way to do it, would have gotten some interest rate break for that. But at the time of initiation, we would not have been able to afford the higher payments of a 15 year loan. As it turned out, the 30 year with prepaying allowed us to boost up the Principal payments when we could, and ease off if we needed to. With the higher monthly payment of a 15 year, we would have no choice but to pay the increased amount due every month.