In any case, in all institutions, you will have to take tests, go through interviews and have your academic resume analyzed. For all selection processes, you can take specific preparatory courses, that is, courses aimed at both the career you want and the desired institution. For example, it is possible to take a specific English course for master's degree exams.
There is also the possibility of special lead taking courses as a special student. This option will help you create your preliminary project, and the credits may be valid for your master's degree. In all cases, do a lot of research and exchange information with people who have already gone through these processes. Don't make a hasty decision, as you will spend at least 2 years working on the chosen subject.
2. Get to know the program and the teachers
In order to obtain excellent preparatory results, attend lectures, workshops and other activities that take place at the institution you want to enroll in. Try to meet and talk to professors and potential advisors. Find out about the required bibliography, go to a library and read everything you can. This may even help you define the topic to be addressed.
Currently, the UnB master's program offers 60 course options, as do many other Brazilian institutions. Before applying to one of them, it is important to check whether your research project really fits in with the characteristics of that course. As a rule, institutions try to maintain the same focus or line of research within their selection processes.
And how do you know if your pre-project is well-designed? Does it fit in with what is covered in the course you intend to take? Or would it be better explored in a related area? A good way to check this is through research carried out previously by course teachers.