hi shubham,

At undergraduate level there are three poasible ways of getting admission in iiser.

Give the kvpy exam(sx,sa,sb stream) and qualify it.

Qualify the jee advanced exam and be within top 10 000

If you are in top 1% of your respective central or state board (cut off percentage is mentioned on the iiser website) then you are eligible to give the iiser aptitude test. If you score well in the aptitude test and qualify the cutoff then you get the admission

Now to get in top iiser or iiser of your preference you need to have a good rank in kvpy jee advanced and top score in aptitude test.

P.s I have mentioned this for general category . There is some relaxation for obc sc and st provided

Hope this helped you.