Calculate correct interest on loan when debtor pays late
 

I am carrying a mortgage for someone who purchased property from me. The
problem is that the person paying cannot seem to ever pay on time. I built
the amoritization chart for on time payments, but need the formula for
properly posting the payments, and ensuring that I post the interest and
principal correctly. Sometimes the payments are up to 60 days past due.
Does anyone have this built already or can provide me the concept?