For international students, the most important thing is the level of Japanese. Can listen and speak, at least level 2, of course 1 is better. See what department you want to enter. It is estimated that business is the most difficult (this is the valuable part of the extension), engineering is also difficult and the tuition is expensive (please think twice), and politics and economics (which should be the weakest part of the extension, but unfortunately my brother is here) should be the easiest to extend. As for the interview, just ask the teachers in your own school, and they will tell you what to prepare. Usually there will be a mock interview.