As you said, you know NCERT class 11 and 12, which is a positive sign and almost 80% of the preparation is done with this. Now if you have completed your syllabus, then do a revision. If you find anything doubtful, try to clear it anyway. As you have still 4 months left, and this is enough time to prepare for COMEDK. You can also prepare short notes which will help you during revision. Write down all the formulae in a notebook so that you can access it anytime you want. Once done with the syllabus, move on to completing Sample papers and previous year exam papers. Go through the link for sample papers https://engineering.careers360.com/articles/comedk-uget-sample-papers Some best books for preparation of COMEDK-

• Objective Mathematics by RD Sharma
• Physical Chemistry by PW Atkins
• Chemical calculations by RC Mukherjee
• Concept of Physics by HC Verma

These books are recommended by experts. These books have large number of exercises and questions for competitive entrance exams. Once you complete the syllabus and revision, give mock tests regularly. Giving mock tests will boost your preparation level and will give you more confidence.