Sorry for the late reply! By upside down I mean - first you scan it normally, and then you turn the paper around (but still keeping the painted surface down), so once it's scanned, you have two files: one is as it should be, the correct way, and the other is turned bottoms-up. Then you can open a program like photoshop or gimp, put both those pictures as two layers, flip the 2nd one to the proper direction, make sure they align well, and set the 2nd's opacity to 50%.