If you have a basic handle on a GUI Bitmap editor such as Photoshop or GIMP, and you have a hi-resolution phone, you can just take a photo of the sheet as parallel as you can manage and then create a document that is the same dimensions and then use the warp tool to fit the likely skewed photograph to the exact digital document.
ha! As soon as I wrote that I thought "there's an app in that" and started mentally mapping out how it could work. I figured it'd need to be simple as possible, and even considered using the user's location to preëmpt the paper format choice :)
