First, for getting admission into any of the IIMs, you need to obtain a good score in the Common Aptitude Test (CAT) examination. Once you get a good score in the CAT, you will get calls from different IIMs to attend their Written Ability Test (WAT) and Personal Interview (PI). You will be shortlisted by the performance in the Personal Interview and your Essay. Other factors also play an important role in the overall decision such as the following.
Each IIM has its own weightage for each factor. Thus, for getting in any IIMs, the performance in the CAT exam, WAT, and PI is very important.