Ghost and story world are probably the most easily distinguished of the nonessential steps, and they both revolve around establishing narrative framework. The ghost is a figurative one, even though some stories have a literal ghost in them. When you hear ghost, think backstory. More specifically, think of that part of the backstory that haunts the protagonist. Haunting. Ghost. See how that works out? The ghost is something in the hero's past that continues to bother him, a psychological scar.
In Full Metal Jacket, we don't realize the ghost until halfway through the film when we get to Vietnam, and, in fact, the first half of the film at Parris Island is all about the formation of that ghost as Joker and the other recruits are subjected to Hartman's tutelage, culminating in Pyle's murder of Hartman. That murder at the end of the first half is the actual ghost for Joker in the second. It's an even that, in spite of all his wisecracks, Joker can't shake off.
Story world is synonymous with setting. It's the arena where the main action unfolds. This is Parris Island, South Carolina in the first half; at no point in the first half do we leave the training depot. In the second half, it's Hue City, Vietnam. There are other locations in the Vietnam section such as the Marine base and Phu Bai just outside of the city, but Hue is where the main action unfolds.
One think about this main location distinction I'd like to make is this: I wouldn't say that the minor locations are unimportant. Instead, I'd say that they're an extended part of the stage and definitely fall within the boundaries of the story world. You can't just start the second half with Joker going into Hue with Cowboy's unit. You need those peripheral settings to touch base on who the hero is now as opposed to earlier in the story, and to show what leads him to Hue City.
Regardless of the story you're writing, you need at least an inch to lay down those foundations. There's a 2011 post-apocalyptic film called The Day that throws the main characters almost immediately. It's a relatively short film, maybe an hour and a half long, but even that one takes a few minutes to step back and say, "Here are our heroes, and here's how they're related to each other."