To appear for the admission test you should have a valid fellowship (CSIR/DBT/ICMR etc), Online tests are conducted twice in a year (June and December), there are two rounds of interview. Students choices are given more preference and you can even change a lab after a month (if not satisfied). The interview questions are mostly about your research interests and your previous research experience (MSc project, internships etc). I would advice to learn about current projects in CDFD and prepare/write your research interest accordingly.