Effective study notes are short, written in your own words, and organised so you can revise quickly, not a word-for-word copy of the textbook. The whole point of notes is to make revision faster later, so capture key points, keywords and concepts, not full sentences.
Write in your own words. Copying the book teaches you little; rephrasing forces you to understand first. After reading a section, close the book and write what you understood.
Use a clear structure. Headings, sub-points, bullets and numbering make notes scannable. Methods like the Cornell layout (a cue column, notes, and a summary line) or simple mind maps work well for linking ideas.
Use colour and symbols sparingly. One or two colours for key terms and formulas, plus symbols (a star for important, a question mark for doubts), help your eye jump to what matters during revision.
Example: A Class 11 student in Pune reading a long biology chapter writes one page of bullet-point notes with small diagrams instead of underlining the whole textbook. At revision time, that one page replaces fifteen.
Keep formulas, dates and definitions on a separate quick-revision sheet, and review new notes within a day of making them while the topic is fresh.
One practical tip: Make notes you would actually want to re-read. If re-reading your notes feels like re-reading the book, they are too long. Aim for short, visual and skimmable.