If you intend to make a separate “Principal Only” payment with your regular payment, send in the “Principal Only” payment first.
The reason I say send in your principal only payment first is because your mortgage servicing company will almost always apply the portion of your payment containing the interest first and your principal payment afterwards for a good self-serving reason. It’s a wee bit self serving but it’s not illegal. It gets them extra revenue simply by holding your “Principal Only” payment.
If the principal only portion is applied first, the amount of interest owed declines slightly based upon the size of the principal only payment. Admittedly it is only a small amount unless you make a huge principal payment at a very early stage of your loan.
Anyone having questions on this explanation can ask them; I’ll explain. On this one I’d prefer the questions (if any) be asked publicly on the MSF forum so that others can benefit.