Okay, time for words!
First things first. Get a piece of fabric. I'm using a nice stiff outdoor fabric that I got from the value section! If you use something more like cotton quilting fabric, you might want to use some interfacing to stiffen it up.
You will also need an iron, an ironing board, D-rings, and quilt binding (the cotton kind that is about 3/4 inch wide), a needle and some thread.
I folded the fabric in half. It was a squarish shape to start. I didn't bother measuring or making sure it was really a perfect square because, after all, I was just going to fill it with dirt, right? So fold it in half and iron the crease in. I forgot to take a picture of that part...
Next, open the fabric up and fold the edges to line up with the crease you just ironed. Basically, you are dividing the fabric into fourths from top to bottom. Iron those creases in, too. Like this: