applying payments to invoices?

[Jeremy Weiss]

I'm trying to figure out how to apply payments to invoices and what to do
when I have one payment for multiple invoices, partial payments, and
multiple payments per invoice. I've been looking at this for so long now
that none of it is making any sense any more. Does anyone know of a good,
detailed explanation on how to do this; or possibly know where I could get a
sample database with this functionality in it so that I can see how this is
suppose to work?

-jeremy