Do you have a plastic square ruler made for rotary cutting fabric? If you do, most of them have a 45º line on them that you place on the sewn line and then trim around the edges, making sure you have the 5-1/2 inch measurement under the ruler. If you don't have a plastic ruler, you can still trim your block but it will be more difficult. Take a piece of cardboard (cereal box) and measure and cut a perfect 5-1/2 inch square. Place your cardboard square on your block, lining up the two corners of the cardboard square with the sewn line edges showing on the corners. Mark all around the cardboard square with a pencil and then cut out your 5-1/2 inch block.