Go to the Town Hall and select the QR Code option. There you are able to create your own, (which will save to your DS Photos) or you have the option to scan QR Codes. To do this, I usually find a QR Code of a mii I like here on miiverse and take a photo of the QR Code using my phone and then scan the code to my 3DS. BUT, there are tons of other methods you're able to use too, not just this one!