It all depends on your threat model, what is your fear? I personally would be very comfortable noting stuff down on a notebook, or even having a random text file on my PC, neither of those is likely to get compromised/tampered. Let’s assume you have someone living with you that you can’t trust, and you don’t want them to either be able to alter or read your entries, notebook and text files are not enough, but you can encrypt the notebook using any multitude of ways (including inventing your own language and symbols) and you can password encrypt the file in your PC. They could still destroy entries or the entire thing, if that is more of your concern then having backups might be more important. If you’re worried about altering past entries you can use something similar to a Blockchain, where the hash of your previous message is used in the new one so it’s obvious if someone erased a message, in the notebook you can do something like starting each message with the 5th to last word from the previous one or in some other way reference it.

At the end of the day it all depends on what is it you’re afraid could happen to your entries, so we’ll need more information on that.