If you are asking about BS-MS program then you have three options to go into IISER's :

• Kishore Vaigyanik Protsahan Yojana (KVPY)

• JEE Advanced

• State and Central Boards (SCB): For this you have to be the top 1% of your respective board.

Now for IPhD program you have to visit the link provided below for the eligibility and proforma of application:

https://www.iisermohali.ac.in/home/front-page-articles/admissions-to-intphd-program-august-2021

For PhD program you are required to qualify the CSIR NET examination and you can follow the link below to know about the details of application:

https://www.iisermohali.ac.in/home/front-page-articles/admissions-to-phd-program-august-2021

I hope this answer helps. All the very best for your future endeavors!