TL;DR: Install it like any add-on car mod. Drag into your dlcpacks folder, list in dlclist.xml

Extra details:

Drag the demonv8 folder into GTAV/mods/update/x64/dlcpacks
Edit the dlclist.xml in GTAV/mods/update/update.rpf/common/data
Add the following line - <Item>dlcpacks:/demonv8/</Item>

To use on any car, edit the vehicles.meta it is defined in, and replace this line:
<audioNameHash>demonv8</audioNameHash>

Questions, comments? Contact me on Discord: https://discord.gg/8mTXGpZMnz