Basically what he said to do is to design a background for your tut and everything the canvas.. then go to your sig and like do your first step and that would be to move the canvas size which you would just drag over a black or white square, etc of the right size. Then just keep going.
I usually do what scr said but instead of merging layers I just hit apply image and drag that over just in case I wanna go back and see anything I may have forgotten to mention.