Instructions
Log onto a "Sims 2" fan site with user generated mods such as The Sims Resource or Mod the Sims.
Click the "Download" button on the menu bar.
Select "Sims 2."
Type "Demon" into the search box.
Scroll through the list of demon mods to find the modification you would like to install. Some will include skin, eyes, hair and clothes, while others will include only one feature.
Select "Download" from the mod menu and save the file.
Open up the "Documents" folder. Select the "EA Games" folder and the the "Sims 2" folder. Move the files that you just downloaded into the "Sims 2" folder.
Open "Sims 2" and select "Create A Sim." Your new demon options will show up on the list of options for your character.