Fill down principal payment & interest payment formulas in Excel

I am creating an Excel table to show the amortization of a mortgage loan in Excel.

For the principal payment (the amount of the payment that is going toward principal) column, the formula is:
=PPMT(B1/B3,__,B2*B3,B4) where __ is the value in column A and in the same row as our formula. So, that's the only thing that is changing in our formula. The other cells that we are referencing in our formula need not change from one cell to the next, as we move down the worksheet in our column.

Whatever answer I get here I will also apply to the interest column in Excel.

Thanks for any insight you can provide.