I do this process in code, actually, by using a somethat TDD-like methodology of "scoping" through the levels of code, where I flesh out the function signatures

I do this process in code, actually, by using a somethat TDD-like methodology of "scoping" through the levels of code, where I flesh out the function signatures of all the classes, write the high level stuff like how these classes are coupled (and discover I missed some passing of objcts/values/data/control flow) and then the rest is just typing. In almost all of these cases there is a lot of thinking involved before typing ANYTHING where I take 2 or 3 days after first looking at the problem to think about different aspects of the problem and that sub-problems they might have.