The walkthrough is downloaded separately. It's always at the bottom of the download choices (for itch at least).
Viewing post in I can't get the special missions c a chiyo
It's not in the game itself. It's a .pdf file that is in the folder of the game files after you unpacked it from the .zip. The only exception is the android version.
And yes, an alternative way to get the walkthrough is to select the download choice at the bottom of all download options here on the game's page on itch.