Hi Raj! IISERs have an integrated PhD programme for the students who aspire to pursue master's and PhD from IISER and have completed their undergraduate degree from elsewhere. The applications are invited once an year in the month of april and the notifications and guidelines are laid down on their website. You can try applying there, you may get an admission if you have excellent grades.

Another way of getting into IISERs for the 5-year BS-MS program is through three chaneels namely- KVPY, JEE, SCB. Again your grades and performance will solely decide your admission.