Hi,
When you start out with a 2.5” square, you need two additional 2.5” squares for the outer square. Cut each square once on the diagonal. This yields 4 half square triangles.
sew these 1/2 square triangles onto your square.
when it comes time to trim, you need to trim more than the “rabbit ears” off.
line up your 1/4” line on the point of the inner square, then trim.