Usability plays a role in each stage of the design process. The resulting need for multiple studies is one reason I recommend making individual studies fast and cheap. Here are the main steps:
1. Before starting the new design, test the old design to identify the good parts that you should keep or emphasize, and the bad parts that give users trouble.
2. Unless you're working on an intranet, test your competitors' designs to get cheap data on a range of alternative interfaces that have similar features to your own. (If you work on an intranet, read the intranet design annuals to learn from other designs.)
3. Conduct a field study to see how users behave in their natural habitat.
4. Make paper prototypes of one or more new design ideas and test them. The less time you invest in these design ideas the better, because you'll need to change them all based on the test results.
5. Refine the design ideas that test best through multiple iterations, gradually moving from low-fidelity prototyping to high-fidelity representations that run on the computer. Test each iteration.
6. Inspect the design relative to established usability guidelines, whether from your own earlier studies or published research.
7. Once you decide on and implement the final design, test it again. Subtle usability problems always creep in during implementation.
Don't defer user testing until you have a fully implemented design. If you do, it will be impossible to fix the vast majority of the critical usability problems that the test uncovers. Many of these problems are likely to be structural, and fixing them would require major rearchitecting.
The only way to a high-quality user experience is to start user testing early in the design process and to keep testing every step of the way.