OK, i didn't read the whole thing, I'm not sure at which point you are getting stuck. But it looks like they want you to make a work axis down the vertical center which you will probably use to revolve around after making a sketch. Then it says to pick a square face and create a sketch on it. Then you want to slice graphics because the sketch plane falls inside at least some of the part you already made. This will allow you to see fully what you draw on this next sketch, and not have it be hidden by the existing material. It's just a graphical peek inside to see what you're doing. Then it says to project the geometry of the existing part, so that when you start sketching you have reference points you can use. The outlines of the existing geometry then become an active part of the new sketch and you can use them in your next extrusion. I hope that helps explain a little bit of why you're doing those steps.
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