How do I get 70/70 in physics in the class 12 CBSE board exam?

First of all, Good luck for your exams, and keep in mind, if you have studied with focus and along with enjoying the subject, scoring is not a big deal. I loved Chemistry in school, and never had any problem in scoring less than 99% in any exam. because I used to have fun studying it, so never lost my interest nor focus.
Now as per me answering, I got 95 in CBSE’13 in Physics, my lowest :p. But keeping that in my I can be the one who can answer you what went wrong for me where I lost marks, or else I could have secured 99 in Physics as well.
Now first How to prepare, NCERT is the mother of all if we are in CBSE and hope to crack any competition in India, JEE or IAS as well. So, for boards, do a thorough NCERT read and make notes of important points. Keep a notebook exclusive for the points you find important while your reading, because if you have deemed them important, you won’t forget them easily if you revise it just once.