What are good ways to prepare for BITSAT if you have 8 months left for your exam?

Yes, it is possible for you to crack BITSAT without joining any coaching center.  First of all read NCERT thoroughly. as you said that that you were not serious during last one and a half years, try to get flying colors in board examinations too as you have to score above a certain cutoff percentage to get admission. BITSAT generally has around 150 questions that you have to solve in 3 hours, though you may also for 12 additional (bonus) questions if you have finished those 150 before given time limit. as it is obvious, since the number of questions are already very high, they won't be much lengthy. usually questions can be solved by using simple logic, straight away formulas and general ability. but one thing should be very clear, and that is your concepts. one more tip that has nothing to do with question paper setting and level is stop wasting time on internet. you would have plenty of time to do that once you enter a good engineering college. I got AIR 7800 in IIT JEE and i blame internet for it up to a large extent. again,manage your time properly and keep interacting with your professors from time to time if you have any doubt. GOOD LUCK!!