how to study after boards exam

I am assuming you are appearing for IIT JEE as you have not mentioned in the question.

FIrst thing, for JEE Mains which is just after Boards
You can start with formula revision. ( 1 day each subject)
Then go for papers daily and most importantly after every paper you give analyze your mistakes and improve on those.

After JEE-Mains there is gap of 1.5 months approx. In that you can work on concepts which you feel you are weak at. Prepare weeklly targets and make a proper revision plan for 1 month. Make sure you categorize your strengths and weakness and devote time accordingly.
Last 15 days solve as many papers as you can. Analysing is most important again as giving papers doesnt help until you get your mistalkes rectified.
Just calm down 2 days before the exam, Keep you r energy positive. Do things which keep you fresh before the night of exam.