All NCERT (National Council of Educational Research and Training) books are not relevant for UPSC (Union Public Service Commission). Hence, you should be selective about the ones you read.
Your first task is to make a list of all such relevant NCERT books. How do you make this list? Simple, look at the syllabus—pick up NCERT books for all the subjects mentioned in the prelims syllabus.
Once you're done preparing a list of the books to read, start reading. Read the history, geography, civics and economics books of classes six to ten, at least once. Read the history, geography, sociology and psychology books of classes 11-12.
Read books subject wise: Read them subject wise, not class wise. For example, finish all Geography NCERT textbooks (sixth to 12th) first, then go for polity, then economy, so on and so forth. This will help you develop a good understanding of one subject before you move on.
First reading: Do not underline or make notes in first reading. This round is just to ensure that you get a sense of the overall subject. We will come back to the underlining and detailed reading later. Next, look for previous year questions of prelims and mains. Try to solve them.
Second reading: During the second reading, lead with the PYQs. Instead of simply reading the chapters, focus on finding areas where the PYQs were asked from. You can underline important areas in this reading. Also, at this stage, you need to start googling concepts you don't understand.