How to Turn a Square Border Into a Round Border
- 1). Hold the penciled end of a protractor on the corner of the border and the sharp point on the other leg about 2 inches toward the middle of the shape. Hold the top of the protractor straight up. Place a small piece of cardboard under the sharp point so you won't damage the surface of the border.
- 2). Adjust the penciled leg of the protractor to bring it in slightly closer to the other leg. Rotate the penciled leg of the protractor to the top edge of the shape by turning it on the sharp end on the other leg without lifting it.
- 3). Press the tip of the pencil to the top edge of the shape and turn it downward until it reaches the adjacent edge of the shape and forms the rounded corner. The distance from the corner of the 90-degree angle should be the same from the start of the rounded line as it is from the end of the rounded line -- this ensures that the rounded corner is even. Keep the protractor at the same angle and do the same thing on the other corners of the shape. Cut along the rounded lines.
Source...