When building the user interface for a JavaFX application, you can do it in one of two ways: code or markup. To keep our code smaller and more readable, we'll build the user interface using FXML--the XML-based language created specifically for JavaFX to express user interfaces. This presents us with another binary choice--do we write the XML by hand, or do we use a graphical tool? Again, the choice is a simple one--we'll use a tool, Scene Builder, which is a WYSIWYG tool originally developed by Oracle and now maintained and supported by Gluon. We will, however, also be looking at the XML source so that we can understand what's being done, so if you don't like using a GUI tool, you won't be left out.
Installing and using Scene Builder is, as you would expect, pretty straightforward. It can be downloaded from http://gluonhq.com/labs/scene-builder/. Once installed, you need to tell NetBeans where to find it, which can be done in the Settings window, under Java | JavaFX...