Sewing a finished inner corner edge requires that two inner corner fabric edges are stitched together enclose the corner's raw edge. While sewing the two fabric edges together is fairly simple, if you attempt to flip the application on its face side, you'll notice that the finished corner has a lot of tension. Not only doe sit not lay flat it also does not create a defined inner corner. This is where a simple notching technique is needed to complete the corner application.
If you are at the stage where you have already sewn your inner corner seam and are ready to flip it to its right side, apply another stitch directly overlaying the existing stitch at about 1" on each corner as shown above. This added stitch will provide some extra reinforcement and strength to the corner once it is notched in the step to follow.
To release tension in the corner seam and thus be able to flip it on its right side, slash the seam allowance diagonally at the corner stoping right before the corner stitch. Be vary careful not to accidentally cut through the stitching. This simple diagonal slash will now allow you to spread the layers apart, eliminating tension in the seam.
The finished inner corner should now be very easy and intuitive to turn on it face side. To complete the finished edge iron it flat for a professional application.