You have to talk to Sable (The hedgehog in the Able Sisters store by the sewing machine) 10 days in a row. After you do that she will put a QR machine by her. There is where you can make and receive QR`s
If you have their QR Code, then when you're at the machine, you can "Read QR Code" and point your 3DS cameras at a QR code (so point it at your computer monitor, or your phone screen). Then the code will be added to your Patterns list in-game, and you can tap the pattern and wear it as clothes.
I think you can put it in your Able Sisters, and your friends can come and put it in their patterns, too. But you can't make it an actual shirt item to drop or mail to others, it only exists in your patterns menu.