What you look for first is always a unit that functions on its own. Then you program that. For example, a feather could be used in a feather bed, or it could be

What you look for first is always a unit that functions on its own. Then you program that. For example, a feather could be used in a feather bed, or it could be used in a duck. You can program a feather, test it and make it work, without needing to have designed or modelled a duck yet - of course, you'll have an idea where it belongs, but since it is an enclosed and reusable component, it can be created first without a top down model.