When you load the game you won't see the standard splash screen or background screen. Instead, you'll be greeted by a 3D window and a toolbar. Under the Tools tab, select Particle Editor to load Particle Editor tools:

Create a new particle system by clicking on File | New. You will see the view has now changed dramatically.

Welcome to Particle Editor! The top-left window lists all the particles within *.pcffile (the file that contains the particle scripts) and it also lets you create, copy, and delete particle systems. The top-right window is an engine preview. If you don't have a map loaded, you will not see anything. Press the F10 key to switch to the engine view, and then press F11 to make it full screen. You can now load any map you want. At any time, you can press F11 to minimize the window and then press F10 to go back into tools mode. The bottom-left window is where we'll be spending most of our time; it's where all the particle properties are set and tweaked....