Initially, you will start studying hard but as time passes you will lose steam. Or you would study a subject and like it so much that you keep studying it for months, ignoring the rest. The risk of going astray is high if you study haphazardly. Therefore, you must have a proper IAS planner to guide you.
So in a marathon exam like the UPSC civil services exam, creating a foolproof study plan is very important. And more importantly, sticking to it, is very very important!
I found a very good answer for question: IAS Planner 2020 for Beginners
