Need definition on
1. What is usability study?
2. What we want to achieve as part of this study?
1. A usability study is simple. All you need is a design, a person who would normally use it, and a place where the person can try out the design. Optimally, the designer observes the person using the design so you can learn about behaviors in the interaction and how well the design performs.
You should be doing usability studies or tests throughout the design and development cycle. Start on rough prototypes (even paper and pencil drawings can get you useful feedback). The important thing is that you're watching people and listening to them as they use a design. You're not asking them how they feel about it. Ideally, usability testing is part of an iterative design process.
2. The point of conducting usability studies is to identify things about the design that frustrate users and prevent them from doing something they want to do. By observing people using the design, you can see where the design does not match the user's goals and activities -- and *why* that's happening -- and then make changes to the design to smooth out that interaction.
Disclosure: I am the author with Jeff Rubin of Handbook of Usability Testing Second Edition. You should buy it. You should also see my blog at http://usabilitytestinghowto.blogspot.com/.