yeah but in that case, the person the diary belongs to might forget how to decode it too. so back to square 1.
That's easy: just read through your own handwriting, until you stumble onto 12 words that form an incoherent sentence. It might take you a few hours, but you'll find it.Quote
whenever you try and obfuscate information inside other information, you run the risk that you forget the encoding process and how to recover the information.
That's true. And that's why I wouldn't suggest adding your own encoding, just keep the 12 words together. I'll give you a hint: search for "Blanket" on this page 
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a diary composed of list of meaningless sentences, sounds like a bitcoin private key hiding somewhere. 
Well, I couldn't find a meaningful diary to download as an example 
In real life, if you have a diary already, you can just add the seed words today in the middle of your story. The coming months it will be hidden "deeper".