Sewing a mitered corner involves several steps, including measuring and cutting the border fabric and attaching the fabric to the project. After this, measure, mark and pin the mitered corner, sew the corner and press the seam.
Measure along the length of each side of the project to determine how much fabric length is needed. Include an extra 6 inches on each end for seam allowance and the creation of the mitered corner (the total length measurement is the length of the side of the project plus 12 inches). Do this for all four sides.
Securely pin the border to the project. Once all four sides have been pinned, sew the border onto the project, leaving 1/4 inch (or the length of your seam allowance if it is more than 1/4 inch) at each corner.
Create the mitered corner by folding the project in half corner to corner (top side in) to make a triangle, and line up the border fabric, one directly on top of the other. Using a pencil and ruler, follow the edge of the folded project and mark the border fabric (on the back side); pin securely and accurately.
Following the traced pencil line, sew the border fabric together. Remove pins and open the project to check for gaps in the corner and to ensure the border lies flat. Trim the excess fabric from the corner. Repeat for all corners. Press the seams flat.