Whoa, so useful! Thank you so very much! Though it took me a while to find out how to do the first step. I searched around and just used the slice tool though. (Is that even how you're supposed to do it...?) Anyways, thanks so much!
You know setting up a Photoshop action after recording will do that for you once you press play? You pretty much just repeated what was said in the tutorial, but "Paste in Place" removes the need to re-position it manually
For those that don't know, you can use the crop tool to get the guideline dead center. Select the entire canvas with the crop tool and the guide will snap to the center of it, then just press escape to exit the crop selection.