Right-click Derealization-0.0.4-pc.zip and select "Extract All..." to a folder of your choice.
This specific .zip is for Windows ; however, the developer also provides versions for macOS and Linux on Itch.io . Tips & Troubleshooting Derealization-0.0.4-pc.zip
To play the game using the .zip file provided, follow these standard steps for Ren'Py games: Right-click Derealization-0