How can a software tester improve their note-taking skills while doing exploratory testing?
Should we take notes in a simple text file or should we take it in a fancy Excel sheet?
How can we take effective notes that can be helpful while revisiting a testing module or application?
It really comes down to who is consuming the information contained in your notes and what that information will be used for. If your testing found a bug, for example, you would likely want to write up steps to reproduce, information about your environment, and other information that would help find and fix the bug. If you are reporting on what you tested to management, you might jot down things you tested, things you didn't test, and potential problems you found. If the notes are for your own purposes, another thing you might want to include are ideas for future testing.
As for format, again, it really comes down to what is most helpful for you and your consumers. If you find that an Excel spreadsheet or some other template is helpful for organization, by all means, use it! On the other hand, if there isn't a need for a lot of formal documentation, you might find it's better to keep it simple and find a format that provides value without imposing a lot of documentation burden.
Here are a couple of resources that might be helpful: