Usability is not an afterthought. User-centered design is all about making it meet the needs of your users, and the process starts at the beginning. A design project will usually start with some kind of business or organizational need. "We need to build a new (app, site, whatever) because we need to increase revenue, provide something new, or whatever."
Your application will go through many user-centered process if it's designed properly:
- if you can speak with typical users of the app during needs assessment and requirements gathering then that's a good idea
- the personas or user archetypes you refer to will be part of this process - if you can round your users into typical types it helps you develop use cases that apply to one or more of them
- depending on what you're building you may/may not be able to test visual designs, mockups, or wireframes on them
- at the very least you'll have early prototypes you may test on your users and high fidelity prototypes you pretty much must test with your users (this is about seeing if the design decisions you made to translate requirements into a design actually meet the needs of your users)
- having iterative cycles of prototype testing allows you to test/refine/test/refine until you get it as close to right as you can
Often if design only focuses on business requirements, it risks the possibility of not meeting users need, and possibly even being completely unusable. The bigger the budget and the higher the risk, the more imperative several iterations of design testing are.
Only when you've got to the point where business and user needs are in a reasonable balance, can you really put things together and build a beta or pre-launch version of the app. If possible you should still be testing pre-production prototypes with your users at this point.
All of the above is before the designed application goes live.