There is no superpower required to score more than 90% marks in CBSE board class 12th exam. But you have to follow the common path, they are:
1. Read NCERT book end to end and make a strong grip over it. As nothing in boards comes out of it. You can also read and download class 12 NCERT books online.
2. Make suitable notes of each chapter so that it will help you in quick revision. At this time if you have not prepared notes then don't waste your time in preparing notes go for online notes. In mathematics section, revise important formulas and theorems. You can get Math formulas for class 12 here.
3. If you are done with the above aspects than its time to solve questions. First, solve CBSE class 12 sample papers issued by CBSE board officially. Then solve at least previous 6 years board question paper which will boost your confidence level.
If you follow these steps with having good management of time then without any doubt you can score more than 90% in CBSE class 12 board examination.