Do not align your paper against the edge of the scanner, doing so will cut off the edges of the scanned content.
Scanners tend to have two corners of the actual scanning bed that cut off a lot more than the opposing sides. We can use that space for materials to help straighten our scans as it’s lost space anyway, and we want to make sure we have the most space possible for scanning while also keeping things as straight as possible.
For example on the CanoScan 9000F Mark II the left and bottom sides of the scanner cut off a lot more than the top and right. If you’re using a different model you’ll need to do some test scans to figure out which sides it cuts off more in your case.
These should only be used when scanning boxes or other thick items. One recommendation is the SHINWA CARPENTER’S SQUARE 30CM*15CM Model No. 12130. It does require a trim on the 30CM side to fit the flatbed which is kind of tricky as it’s stainless steel. However if you have the tools to do that it’s a great solution as it’s thinness fits within the scanner well.
Another option that’s easier to trim down but isn’t as easy to lineup as a square is CARL RM-12/2 12-Inch Cutting Mat for all 12-Inch Trimmers.
Example picture of Carpenter Square & Band-Aids:
Now place your square/rules along the corner of your scanner which you know has more dead space, press your flattened boxes up against its corner to get a straight scan, and lower your flatbed lid. Place a heavy book or object on top of the scanner to flatten the box as much as possible and hit Preview on your scanning software. If the scan looks straight go ahead and scan it! If not readjust and preview again until you’re satisfied.
Playing cards and index cards pressed along the sides of the scanner, can help with paper thin materials such as manuals. If you are going to scan a lot of manuals, and have extra funds (as they are not cheap) it might be worth looking into an Automatic Document Feeder (ADF) scanner.
Another important point when scanning a manual, consider removing the staples for a higher quality scan. Again place a book or something heavy to get the scan as flat as possible, and do a preview before scanning to make sure the manual pages are straight enough. These are notoriously hard to get straight so just do your best.
Dealing with bleed-through is another thing you’ll have to deal with, especially when scanning in white manual pages as they’ll usually show text etc from the opposite side. An easy solution to this is to use some black construction paper or posterboard and place it on top of the thing you’re scanning. One way to rig it up can be by trimming a sheet of posterboard that fit your scanner plate, and then tape it along the top of the scanner. Your scanner might looks a little weird with tape on it but it works!