Mini Tutorial: Sewing opposite corners
If you attempt to sew opposite corner edges together you will most likely be perplexed by how it is physically possible to align these edges into a functional seam. In this case, a simple preliminary step is the answer to what seem like an impossible task, especially of you are a sewing beginner.
In order to be able to align an inner corner to and outer one, all you have to do is simply slash diagonally into the corner up to but not through the seamline. To maintain the corner's durability and stability during this notching process, it is important to add a machine stitch along the corner seamline, extending at about 1" on each side (as shown above)- this is called staystitching. When clipping the corner, simply cut up to this stitch, but be careful not to cut through it.
Like magic, the inner corner diagonal slit will now allow you to spread the fabric layer apart and match the corresponding edges. Insert pins perpendicular to the edge to keep the edges aligned in the stitching process.
When stitching the opposite corner seam, you should be careful when pivoting the corner stitch as this will play an important role in achieving a well defined corner seam. Stitch up to the corner and insert the needle right at the corner point, staying to the left of the staystitch applied in Step 1. With the needle inserted, lift the presser foot, shift the fabric layers and continue stitching to complete the horizontal seam.
To complete the opposite corner seam, make sure to trim any excess seam allowance to eliminate unnecessary bulk. Flip the seam on it right side and iron the completed seam for smooth, professional application.