Solve Arihant’s BITSAT book completely. Most importantly solve previous years BITSAT questions as you will get an idea of what kinds of questions can you expect. There are some good mock tests on HTCampus. Solve them if you have time. Time is an important factor in BITSAT. So while attempting mock tests, experiment with various strategies and choose the one that best suits you. Identify your strengths and weaknesses and strategize accordingly.
Since you are weaker in physics and chemistry, it would be better if you devote more time to these than to maths. Questions of physics and chemistry are easy and you can score easily in those two if your basic concepts are strong. NCERT will help here.
Practice as many questions as you can and be thorough with NCERT. Also, DON’T ALWAYS go for those 12 extra questions. You will get a hang of when to go for the 12 extra questions and when to not while you practice mock tests. So don’t worry about that.