How to use this plugin
First of all, the plugin needs to be installed.
The plugin only shows a preview for supported languages, such as HTML and XML, otherwise it’ll just say "(No preview available)". You can change the "Language Mode" for every document in Notepad++'s Language menu.
You can add support for other languages to the plugin by defining filters.
Open the preview window
To preview a document, you can either press the preview icon on the toolbar, or use the menu item
Plugins / Preview HTML / Preview HTML, or the associated shortcut key. The preview now ought to appear in a window.
If the preview window does open, but doesn’t show an actual preview, try hitting the Refresh button. Otherwise, try docking/undocking the window to make the contents appear. I’ve heard some people have had trouble with that.
Docking/undocking the preview window
The preview is shown in a so-called ToolWindow. This means it can be ‘docked’ inside the Notepad++ window, or dragged out as a separate window. The easiest way to switch between docked and undocked, is by double-clicking the preview window's title bar. You can also drag and drop the window by its title bar; the window can be docked near the bottom of Notepad++’s main window, on the left, or on the right side.