First, make your paper square. You can do this by folding the paper in half at a diagonal, aligning a side edge to the top edge of the page. Tear or cut off the excess strip.
If you want a top for your box, make one from a slightly smaller piece of paper. You'll be able to slide it into the first box upside-down. It's also neat to make a whole set of nesting boxes in multiple colors and patterns. Or just make one, and put a little stuffed dog in it!