Hello Student

You have to practice last 10 years JEE mains and advanced question paper and IAT solved question papers. Most of the IISER questions is same to the JEE main and advanced questions and there is no PCM questions at all which one find in the JEE advanced examination for IIT. You have  to buy 41 years pyq books chapter wise. So there is no need to join any institution.Only to practice the above-mentioned books and questions to get a good result in IISER .

Hope this helps.