Hi

If you are preparing for JEE then preparing for your CBSE board examination becomes a little bit easier because Jee syllabus goes well along with CBSE syllabus the only difference is that your Jee examination will be objective while your CBSE board examination is going to be subjective but yes you will have to manage time because for JEE preparation you'll have to practice objective questions while for CBSE Board Examination you'll have to practice derivations etc.

To score 90% or more you need to prepare well for your board examination which you can do following  the below tips :-

*First of all go through NCERT and don't ignore it at all most of the questions in board examination are based on NCERT, you can take the help of reference books to understand the difficult  topics and have better understanding  but major focus should be on NCERT

*make a routine and religiously follow it, it will help you to avoid wastage of time and study in a systematic manner

*For mathematics, practice is the key , the more you practice the more you get good at maths. Focus more on miscellaneous exercises of NCERT Maths , go through all the examples as well.

*For chemistry go through all the solved examples and intext and exercise questions of NCERT , in organic chemistry focus more on name reactions there are two questions from name reaction each year in board examination and many questions are based on them directly

*For physics,  practice  all the derivations from time to time as there are chances of forgetting them if you don't revise them, derivations are very often asked in your board exam, go through solved examples and then go for unsolved numericals.

*Make short notes of important concepts and formulas for each chapter

*Go through previous 10 year board examination question papers as concepts are often repeated in board exam so it will be of great help in your preparation

*Once you are done with your complete board examination syllabus go through at least 3-5 mock test papers in each subject

Thank you