1. If you are a 'below average' student with many backlogs-

Well, you might be feeling left behind and might be getting poor ranks in test(obviously, because you don't know the material well). But don't worry, you can still make it. All you need is a little faith in yourself and hard work. Firstly, you know that you have backlogs. And you HAVE to clear them up in order to well in the exam. So make a list of all the topics you feel you don't have a tight grip on(you can even label them based on how much you know) and take out time to solve their problems. Use Sundays, holidays and free time to clear backlogs. Solve only basic questions(like HCV, the first exercise of coaching class sheet, etc.) first.

Never sacrifice your 12th for covering 11th. Focus well on what is being taught now. and do not repeat the mistakes you did in class 11. Try covering topics like NLM, Ionic Equilibrium, Trigonometry, Straight Line, etc. first, because these are needed in 12th too. Focus on problem-solving. Make sure you manage to clear all/most of your backlogs in 2-3 months. Plan accordingly(as in I have to complete Rotation revision in 2 weeks, Ionic equilibrium in 1 week, etc), and revise all three subjects more or less parallelly.

2. If you are a good student but not very confident of class 11-

Well, you must revise the whole class 11 syllabus again to get a top rank confidence in it. Solve your coaching material again, especially the part which is tough/you couldn't solve it in class 11. You can also go for other coaching material(not advisable), or go for books like HCV, A Das Gupta(I mentioned them in the last post), etc. You must make sure that you revise the entire 11th syllabus(without compromising 12th) in 2-3 months. Solve a lot of problems, give a lot of tests, ask a lot of doubts, and keep on improving your speed.

3. You are really good, and are aiming for a top 100 or so rank-

You guys know it all. You have a very good understanding of everything and are already clear with everything pretty well. There might be a few gap holes. Seal them(like, you might not have studied a less important/few topics. Cover them up well. Then move to the real stuff. Try to improve your skills to get to the top in your tests, prepare for Olympiads and all. Solve Chemistry and Physics Olympiads stage 2 papers (and stage 1 too). Go for other coaching tests. Solve books like Irodov, Krotov, A Das Gupta, SL Loney, Solomons, etc. Whatever happens, do not rest.

Basically, the rules are the same- Work hard and don't give up.

All the best!!!

You can also check out this app I made for helping JEE/NEET/Olympiad/KVPY/NTSE aspirants with courses, notes, blogs, and much more- AcadBoost