Our ConLaw teacher described it this way:
Formalism is defined by the "formal" boundaries of the Constitution. The branches have whatever power they have been given by the Constitution- no more, no less.
Functionalism is kind of like adverse possession of powers. If a branch takes over a certain power unresisted for a long time, then it can be fairly assumed that the other branches and the people didn't have a problem with them doing so. Therefore, we look at how the branches have been "functioning" and what works practically, as opposed to merely what the Constitution specifically states.
This is simple and may not help you at all, but it might do you a little good.