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.