Formed stitched corners from the inside
Tailoring: Using a folder for corner shaping: If you want to form sewn-in corners from the inside, ie you want to evert them outwards, I do not use scissors or other sharp objects, because you can easily "pierce" thinner material (that's already happened to me) ).
For this I use a folding bone, which I once bought for smoothing paper with a bookbinder: It is only slightly rounded at the front and best suited for the corners. From the fabric outside, I carefully pull out the very last scraps, which are still missing at a 90-degree corner, with a pin and give the corner the finishing touch.
Oh, well, point out again that the corners, of course, only "beautiful", if they are not sewn at an angle of 90 degrees, but with a (or thick fabrics: several) diagonal stitch (s) between the long edges, the make the corner. Then cut (before turning) the seam allowances up to 1 mm to the oblique stitch diagonally, and everything looks great!
Something else: If the fabric has been reinforced with fleece-lined, perhaps the corners out of it, so that you can shape them better later ...
