First make a proper and programmatic study schedule which you'll follow throughout your preparation. Then go through the entire syllabus. Instead of choosing a random topic from the syllabus, you must break your preparation into sections. Sub-topics from each section should be selected and then strategy on clearing concepts must be made. Never leave any concepts doubtful or unsolved. Make your own short notes. Once done with the syllabus and concept clarity, you should move towards solving previous years papers. Give mock tests as much as possible. Time management is one more important thing you should look upon. Remember that practise will increase your efficiency.
No doubt, NCERT textbooks of Class 11 and 12 are best for preparation. Along with this, As you told about preparing from RD Sharma is also good. Best books for KEAM- Objective Physics by Arihant, Concepts of Physics by H C Verma, Complete Mathematics by MC Graw Hills, Organic and Inorganic Chemistry by O.P. Tandon.
Some important topics that you must cover-

Physics- Mechanics of Solid & Fluids, Heat & Thermodynamics, Principles of Communication Devices, Dual Nature of Matters & Radiations, Oscillations, Atomic Nucleus, Electromagnetic Waves, Optics.

Chemistry- Thermodynamics, Electrochemistry, Periodic properties, Kinetics, Chemical Equilibrium.

Maths- Probability, Vector, Matrices & Determinants, Functions, Three-Dimensional Geometrical System, Inverse Trigonometric Functions, Mathematical Reasoning.

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