Note also that although the above instruction says ''the major purpose for being a Non-degree research student is to prepare for the graduate school admission'', I would not help you preparing for the entrance exam because it would be problematic from the standpoint of fairness*1. You may find the past exam problems (written in Japanese) in the following website:
I would recommend you not to apply for Non-Degree Research Student unless you find most of the problems easy. Otherwise you should not be able to write a meaningful research plan, and also there is a non-trivial risk of failing the entrance exam. In the latter case, your time and money spent to be Non-Degree Research Student are most likely wasted.*1: I believe "to prepare" in the quoted sentence refers to the preparation for your research, such as doing a reading seminar on a research level textbook, or getting used to life in Tsukuba.