After you place and complete your order for the game's purchase, you should have all the files ready to be downloaded in your Denpasoft account. This step is important: download each file one at a time and place them all in one folder.
For right now, keep the .int file in your Downloads folder, but move the remaining parts into a separate folder as shown below:
Then, open the first file titled "The_Fruit_of_Grisaia_Unrated_Version.part1.rar" by using WinRAR. It should look like this:
Extract that folder to a folder of your choosing. When it's done extracting, move the .int file in to the folder you just created. When it asks to overwrite, click 'Yes.'
Your folder should now look like this:
From this point, run the BootMenu.exe file and click Start. The installation is complete and you can now enjoy your game! If you're wondering why you need to only extract the first part of the .rar files, it's because they're auto-extracted if you select the first file in WinRAR; however, they must be in the same folder for this to work.