This is very simple to achieve. You do need a modem with a button that pauses the connection for a few seconds, though.
All you have to do is find a shop you want to stack, go above it about 1 screen, and scroll down and start walking down a bit until you see ONLY THE FLAG. Click on the flag, it will ask if you want to set up a shop. Now pause your modem with the button on your modem, quickly jump very near the flag, click OK to set up a shop, now you can bring your modem back and if you did it fast enough you will see yourself stacking someone's shop. Otherwise you will DC.
The only reason I'm sharing this is because I can't do this anymore, I'm on a wireless connection now and the modem is 1 floor below.
Because I wouldn't want this to get patched somehow.