Actually, I don't know if I can call it a pattern. There are no measurements involved. I read a blog post lately where someone said they 'eyeballed it', so I guess that's what I did today too.
And before I start, I need to mention that I am terrible at sewing. But, because every cloud has a silver lining, I am awesome with a quick unpick. And I am not going to let my faults stop me from sharing this with you.
First of all, you need to make a little pillow-case like sack. If you can do this without instructions, skip to step 3. Or, just grab a pillow case!
1. Grab a strip of fabric. Cut the long edges with pinking shears and roll and iron the short edges.
2. Fold in half lengthways and sew up the side seams. Hem the top edge.
4. Next, you are going to sew over the corners you created (the red lines in the picture below). This is the 'eyeballing' part that I mentioned before.
5. Once you have sewn the corners, flip the box inside out to make sure they are in the right place.
Looking at the bottom, you can see the 4 seams that you have created:
6. If you're satisfied with it, then go ahead and chop off the corners.
And viola! You have a box!