I mainly agree on this. As dragonchild says, you need to breakdown the system into a small chunks, decide how they interact with each other and document that.

However, I would suggest that you use same tools for documenting and designing the program as everybody else. That should make it easier in the future to read them, since the documentation is more consistent. You don't have to use all the tools all the time. For example if your solution doesn't involve object oriented code, UML most likely won't play as big role as it otherwise would.