You need to stagger your pieces to the left a little bit, so that the triangles on the left and right side peek out a little bit. It looks like the way you have them lined up, you're trying to get a straight edge *before* you stitch them together, when you have to get a straight edge *after* stitching them together.
Look at the diagram just above step 3 and see how the dog ears stick out on the left & right.