The way that the scanners on printers work varies between makes and models. Sometimes you have to have to open scanning software on your PC, and then work from there, but there's often a 'Scan' button on the printer itself.
Assuming that you've got such a button, insert your crossword face down into the scanner. (There's usually an arrow pointing to one of the corners of the scanning area, which tells you that you need to ensure that a top corner of your crossword should be up against the arrowhead). Then press the 'Scan' button. That should bring up some options on your computer screen (or possibly on a screen on the scanner, if it's got one. Select 'Scan to pdf', 'Scan to document' or similar).
You might be given an option to trim the scanning area (by dragging the framing lines in on your monitor) but you can probably ignore that for a crossword anyway. Just click the 'Scan' button on your screen. That will send a scanned copy of the crossword, in pdf format, to the default folder on your PC. (You might be given the opportunity to name it first or, depending on the make and model of your computer, it might just be given a name based upon the date and time of the scan). It might also open automatically in the default pdf viewer on your PC, such as Adobe Acrobat Reader, Foxit Reader or Sumatra PDF. (If you've not got a pdf viewing program on your machine, I suggest installing Sumatra PDF before attempting any scans:
https://www.sumatrapdfreader.org/free-pdf-reader.html . Crap website but brilliant software!)
As I've stated above, the pdf file should be in your default folder for saving such scanned files. That's likely to be in 'Documents'. Once you've located it, you can attach it to an email to the newspaper inthe usual way or, if they've got an 'Upload' button as part of an online entry form, by clicking that button, navigating to the location of the pdf file and double-clicking on it.
If, after having typed all that lot out, there
isn't a 'Scan' button on your printer, you'll neded to find the scanning software on your PC that came with the printer, open it and work from there. (It's basically the same as above).