First step is to study and understand the entire course at least once. If there are chapters you have not done yet, study them at least once so that they are well versed with the full syllabus. You may not do any entrance specific study in the first pass, just understanding the concepts is enough. You can start with important chapters for JEE Main.

Ideally you should be able to complete this step before December ends.

This is the trickiest and the most important part. Spend the time that you are left with after going to school or coaching classes as follows. Pick a chapter (or more if you have more time) from each subject. Revise the basic concepts and learn all formulae and tricks from your notes (or textbook). Divide the time left after basic revision in half and devote to practice entrance level and board level questions.