First, let's look at Pink's puzzle. This is nothing significant, this puzzle basically tells us what ciphers we need to use in order to solve the other ones. We didn't complete it yet, but from what we have, we can tell we need Atbash, Caesar and A1Z26. These are all valid ciphers and the A1Z26 is one that I already know the use of.

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Then we need to check Alan's puzzle, which is hardly completed as we only got two pieces so far, but Gravity Falls fans will know what it means IF they've read the book. It's using the combined symbol substitution cipher, which sounds difficult but isn't really. Each symbol has a number attached to it. Then using the A1Z26 cipher, you take the number and substitute it with the nth letter of the alphabet.
For example, we know the eye symbol resembles 8. The 8th number of the alphabet is H, so the eye symbol means H.


Thirdly, let's get Choco's puzzle. It has the symbols that Alan uses in his puzzle. Using the combined symbol substitution cipher, you can decode the whole page once it's finished. As you get letters from completing Alan's puzzle, you can also make out words by logic.


We only know 'H' for sure so far, but with only this one letter, you can actually decipher words! In this picture, follow this order of colors:
RED > BLUE > GREEN. First read the red text, then the blue one from which you can get to know the third (green) symbol's meaning. It's all logic, so it's worth pointing out that this may not be true! The words from this page could also be jibberish leading to another cipher, which we'd need to solve. Only the H letter is 100% correct.

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We're left with Yari's puzzle, which may be a Caesar-5 cipher, but I'm not completely sure. That means that each letter is shifted backwards five letters. We've only got the phrase 'YMJ' so far. Shift it back 5 letters, and you get 'THE'. It seems right, but may be not!