1) Go to Help > Eclipse Marketplace
2) Search for JavaFX there
3) Install e(fx)clipse plugin
4) Download the JavaFX library from https://gluonhq.com/products/javafx/
and unpack the downloaded archive somewhere (in this tutorial it'll be "C:Program FilesJavajavafx-sdk-18.0.2lib")
5) Go to Window > Preferences > JavaFX > JavaFX 11+ SDK field
and enter the JavaFX library location where you unpacked it:
"C:Program FilesJavajavafx-sdk-18.0.2lib"