Assuming your project is on a DSi/3DS and you have an SD card, from the main menu you can go to "File Management" and then "Save to SD Memory Card". You then export all the files of your project, which will be saved to the system's SD card.

Once you have those, you'll need to copy the files from the SD card to a computer. You then should download PTCUtilities from here: https://micutil.com/ptcutilities/top_e.html. This program has a QR code generator that can create the QR codes from your copied .PTC files (use the "PTC > QR" option).