It's best to apply where there are vacancies. However, even if there is no vacancy and you are targeting to work for the company, do apply. They may call you when there is one. It is better to apply to the partner in charge of the practice group that you want to work for and make sure that you give effort to follow - up with your application status (in a gentle and polite way off-course). There will be one interview or one test or both with the partner in charge of the practice group you want to work for. If you pass this first step, you will be interviewed by the managing partner. It may take a few months from the date of your application to the date you receive the offer letter.