In the left corner of my layout, I build a curved corner with a 33-centimeter (12 inch) radius. By doing this, I create a way to prevent seeing a harsh 90-degree corner, which is more pleasing to the eye and will create better pictures from the layout, once finished.
I use basic materials from left over plywood, 4-ply matboard and drywall mud.
I started by sawing a few corner pieces out of 19 mm (3/4 inch) thick plywood with a radius of 33 cm (12 inch). In the center of the radius, I cut a small opening to attach a small straight piece of lumber to support the matboard.
With strong double-sided tape I attached two sheets of matboard to the wooden frame and with a few nails I secured the matboard a little extra to prevent it from coming loose. However, the double-sided tape is strong enough to hold it.
Now it is time to go at it with drywall mud. Apply, dry, sand, apply, dry, sand and repeat.