From Wikipedia: “Notepad++ is a text editor and source code editor for Windows. One advantage of Notepad++ over the built-in Windows text editor, Notepad, is tabbed editing, which allows working with multiple open files.” 
- Download your plugin e.g. TextFX
- Place the dll file in the plugin subfolder of Notepad++
- Load the plugin by going to Settings -> Import -> Import Plugin and select the dll file.
- place any extra config files within plugin\config
- place documentation files within plugin\doc
For further info goto Notepad++ wiki or their Plugin Central