Tailoring: bone folder to corner molding use: If you want to shaping sewn corners from the inside, so want evaginate outwardly, I do not use scissors or other sharp object because you can easily thinner material times "pierce".
For this I use a bone folder, which I bought for the time smoothing paper at a bookbinder: It is forward only slightly rounded and well suited for the corners. From the fabric outer side I put the last remaining media that are missing at a 90-degree corner, carefully with a pin out and give the corner put the finishing touches.
Oh, so, as once again point out that of course the corners only "beautiful", if they are not sewn at an angle of 90 degrees, but with a diagonal stitch between the longitudinal edges forming the corner. Then pass to 1 mm cut off the seam allowance to the slanted stitch zoom obliquely, and everything looks great!
One more thing: If the material has been reinforced with interfacing, perhaps avoiding the corners of it, so they can form better later ...