This is very easy to do and you must have a shirt or a pattern to do it. Here are steps on how to do it.
1) Grab a shirt or pattern
2) Use your stylis to drag the item to the lower left hand corner of your inventory into an invisible box.
3) Drop the item in the box to get the certain pattern of the shirt or the pattern created. It should show up in the backround and since the fact you have SWAPED patterns, you receive a free Big Dot shirt! Lucky You!