1. Go to G-Senjou no Maou in Steam, right click on Properties, then Local Files, then Browse Local Files.
2. Your folder should look like this:
3. Open the downloaded archive in 7-zip (if you do not have 7-zip, it is available here: https://www.7-zip.org/ )
4. Click the folder in the archive labeled 'G-senjou no Maou 18+ DLC'
5. Copy the 'patch2.xp3' over to the game folder as the readme indicates
6. Your folder should look like this:
7. The install is done!