I got some window decorations from KDE-look.org and I went into settings to change it but it doesn't give me the option to install decorations from file. How do I do so, if it's even possible?
It depends ;)
Engine + decoration files
Example #1- Aurorae - engine included
The Aurorae decoration engine is part of the KDE.
GHNS: System Settings > Workspace Apperance > Window Decorations > Get New Decorations
Manual from a file: Unpack the theme to the ~/.kde/share/apps/aurorae/themes/
Example #2- deKorator - external engine
After the installation of the engine: System Settings > Workspace Apperance > Window Decorations > deKorator
The deKorator has an option to install the theme from a file.