how should i start my preparations for kvpy

Note that the paper/interview weightage is 75/25. So if somebody scores just 5 marks over you in the paper, you'll have to score 15 marks over them in the interview to catch up, which is practically impossible. This is because the range of interview marks is 70-90 no matter how bad/good you are.

I scraped through the written test in SA (silly mistakes) and even if I had gotten full in the interview, I wouldn't have made it! So the next time (SX), I made sure I scored 20 marks above the written test cutoff. My interview didnt go great but still cleared with a good rank.

So in short, your written test essentially single-handedly decides whether or not you clear the interview! So study a lot before the test; NCERT full for chem, mechanics and other conceptual trick questions for physics and lots of practice for math. Bio study the tenth grade syllabus MORE than just CBSE level. I used the Kaplan SAT ll subject test book to study for bio and that helped me score like 5 extra (non-CBSE) marks in bio, which as I pointed out, is enough to make it or break it.

FYI: The only person I know to score above 95 in interview proved Fermat's little theorem USING COMBINATORICS in the ELEVENTH GRADE!!! So unless you are a total genius, DO NOT depend on the interview!!!