Ok, so maybe it's not QUITE 5 seconds, but you'll see. :)
Step One: Find a knit shirt. I choose knit just because I'm lazy I don't have a lot of time and didn't want to finish the edges. I wasn't loving this one from the downeast outlet clearance store, but saw its potential as a cardigan-ish shirt.
Step 2: Fold it exactly in half with the front on top.
Step 3: Cut it. Go ahead.
Step 4: Carefully cut over any seams.
Step 5: Wear it over whatever you want. I love belts.