By your comment that "it's difficult to prepare a windowed bag," I'm guessing you're looking for specific sig placement and not a generic window bag, which of course can be done without a comic for reference.
In the case of a SS book, if I have a raw copy of the same comic, I use that copy to prepare the window bag. Otherwise, I place an empty bag on top of the slab and position to where it would be if the comic were in it, and mark where I want to make the cut out. From there it's just like preparing any other window bag. I use masking tape, primarily because a marker can smear even after it's dried. I learned that the hard way with some sketches I got on comics in window bags that a marker was used for cut out. Probably not as big an issue when you're just getting a sig, because the bag won't be handled nearly as much, but no reason to take that chance. Just be sure not to cut into the masking tape, because otherwise the adhesive from the tape can get on the comic.