Hi mayanak,

Two ways to secure internship at IIT/IIsc : apply for the Indian academy of science program .

It is a prestigious program and it is based on the slection is based on the cgpa ,and recommendation available position according to your available interest .you have to apply for it through theiofficial website.

The other way to sceur internship is to write a email to professors .search for professor who have simliar interest anbdwrite mail to them.

Email consists of 

1) Introduce yourself , cgpa and research interest .

2)Why you want to do particular project ,what made you to choose this project .

3).Read some revelent research paper that professors had published .write a short yet telling summary of that paper and convince them to do particular project .

Thanks