Hi Gaurav,

TO achieve 90+ percent in 12th standard within 6 months is not a very difficult task. First of all target the subjects that you find difficult and work on them. Take up small portions of it and give it time everyday. Dont stress out too much. Weekends can be utilised to revise a major chunk of what you've studied throughout the week and take your time to relax too. Subjects like Maths and physics require a regular practice. Evem chemistry should be revised on regular basis. Just keep working hard and take your home exams seriously. Consider them to be your final aim so that you are absolutely ready till the end.

All the best!

You can easily score 90% even if you start working now. 

Start taking 12th seriously 
I'd just suggest you to mug up the NCERT books first.
Take pre-boards seriously. 
As most of the time, your board practicals marks are awarded on the basis of pre-boards. Even externals also ask you about your pre-boards performance.
Not only this, pre boards will tell you your current status about how much you're prepared for boards.
So,make sure you get 30/30 in three of the practicals.
Now, for theory exams
First and foremost thing is to mug up NCERT books. Get through it. Boards paper constitutes direct questions from NCERT and ask  must know questions.

For chemistry, NCERT book is more than enough. 

For maths, the question paper will surely constitute of 50% direct NCERT questions.
But in maths, you need to do previous year papers. Time management is must in MATHS boards. 

For physics, it is going to be a little tough as compared to other subjects. But if you're good with concepts,90+ is easy to score 
Practice each derivation. Get through the numericals. 

For english, Time management is equally important. You don't really need to use fancy words in writing section. Be direct and underline important points. Do the character sketch for novel section. Scoring well in English requires writing coherent simple sentence that is it.