Download the free iPhone app QR MyDesign Assistant. You can get it on the App Store here. It isn't made for the iPad, but it still works as intended on a tablet.
Choose the 'Convert' tab at the bottom of the screen, then hit the 'Pick' button in the top-right corner.
You can choose 'MyDesign' to make a floor, sign, or item of clothing. Or you can choose 'Board' to make a custom face cutout standee.
Choose a picture from your iOS photo library. Simple and clear images with not many colours work best. You can then crop the image to import just a specific part of the picture.
Here's what it will look like in the game. You can give it a name, and then hit 'to QR' to see your code.
Get the code up on the screen of your phone, and then dig out your 3DS.
Load up Animal Crossing: New Leaf, head to Main Street, and enter the Able Sisters clothes shop.
Now, to gain access to the QR code reader (a.k.a. the sewing machine), you'll need to have befriended Sable. This means talking to her every day for about a week or two (to finally coax her out of her shyness).
Once Sable becomes your pal, you'll see a big old machine in the back of the room (like in the screenshot above).
Done that? Okay, cool. Use the machine and choose 'Read a QR Code'. You'll also need to choose a spot in your texture library to save the custom design.
Then, point the 3DS camera at your phone, and line the QR code up with the frame.
And you're done. You'll now have your photo in your design library, and can use it wherever you want.
Here, I've stuck a photo of Mario on my town flag. Because why the bloody hell not.