Downloaded mods are usually stored in .zip, .rar or .7z archives. While most .zip files can be opened in Windows by default, .rar or .7z require external tools. Commonly used tools are 7-Zip (free and open source) or WinRAR (proprietary).
Models, Textures and Audio Files
Any mods that change original game files, for example vehicles, pedestrians, sounds and textures or models in general are installed using OpenIV.
Script mods usually only need to be moved to specific directories. However, they require different hooks to work. Commonly used hooks include Script Hook V, Script Hook V .NET and RAGE Plugin Hook. Take a look at the script's description or readme file to see which hook(s) are necessary.
Editing configuration files
Many script mods include .ini or .xml files for configuration. Most configuration files can be modified using any plain text editor, for example Windows' default Editor or more extensive editors like Notepad++. Rich text editors such as Word or Wordpad will not work.