I am not sure how big your square was to begin with, however, make a larger square (perhaps 15" square). Decide if you want a 3" inset from the corner. I usually draw a line once I have measured my inset so measure how long that line will be and see if it comes as high as you want it. You can also play with how much you want to fold the top corners over. Some people leave them standing straight up.
I like Paperprincess's suggestion of making a newspaper pattern.