Yes, you can certainly use a card index for note-taking. Card indexes have been a popular method of note-taking for many years and can be very effective. Here are a few tips to get you started:

1. Choose the right size of card index: Card indexes come in various sizes, so choose one that is comfortable for you to handle and store.

2. Use cards of the same size: Using cards of the same size ensures that they will fit into your index box or file and that they will be easy to handle.

3. Choose a system for organizing your cards: You can organize your cards alphabetically, by subject, by date, or any other system that works for you.

4. Use one card per idea or topic: To keep your notes organized and easy to find, use one card per idea or topic.

5. Keep your notes brief and to the point: Use brief notes that summarize the key points of your topic. This will make it easier to review your notes and find the information you need.

6. Review your notes regularly: Review your notes regularly to keep the information fresh in your mind and to make sure you haven't missed anything important.

Overall, a card index can be a very effective method of note-taking, as long as you develop a system that works for you and use it consistently.