Your rotary cutting has to be exact as well as your 1/4 inch seams. I put right sides together on fabric and place one triangle on opposite sides of the square. Pin and sew the seams making sure the tip of the triangles extend beyond the square corner 1/4 inch. After they are sewn, I press open and pin and sew on the opposite triangles.
Pick out your seams, iron the pieces, remeasure and try again. When cutting triangles, you need to be careful the bias edges don't stretch. Kind of tricky beginner pattern in my opinion.