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I tend to design from the bottom up - not exclusively, but in general I make small parts and combine them to make larger parts until I have something that does what I want. I refactor constantly as my understanding of a problem and the solution increase. This way I always have complete working code for some section of the problem. I rarely use stubs of any kind.
To start I will take some small section of the problem and think about what kind of data and operations on the data I need to solve it. For a very simple problem I might just write some functions to operate on the data. As I expand into larger parts of the problem I might find that several functions are operating on the same data and decide that they belong in a class. Or it might be clear from the start that I want to create a class around the data.
When one chunk is done to my satisfaction, I take on another, and another. I am creating building blocks, then using the building blocks to create larger blocks. Some of the blocks are classes, others are functions.
I write unit tests as I go, sometimes test-first, sometimes test-after, but always alternating writing code with writing tests so I know the code works and I have a safety net when I need to refactor or make other major changes.
At any time I may discover that I made a bad decision earlier, or realize that there is a better way to structure the code or data. Then I stop and rework until I am happy with what I have. The unit tests give me confidence that I haven's broken anything in the process. It's a very organic process, I sometimes think of it as growing a program.
(from a post to the Python-tutor list)
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