Back before I knew there was an internet I read the Apple Human Interface Guidelines which it would seem they've been keeping up to date.
You can also read some totally different takes, like Raskin's
So I suppose the answer is read. AFTER you've thought about what you want to do and how, you hire the graphic designers to make it look good while it's happening. But I haven't yet found a designer who more than tweaked the application and I described it.
