Since the tutorial *was* a bit confusing...

 Make an "envelope" of fabric.  (I used paper here because I have no fabric at work.)

 Squish it so that the side seam lays on top of the bottom, where a seam would be, if you'd used two pieces of fabric, and so that it forms a pointy bit.  Stitch across that pointy bit, perpendicular to the seam.

 Cut said pointy bit off, so that you only have a seam allowance.

Ta da!  Boxy bottom on a bag.  Hope that helps!