Ah, okay. The corner pieces contain everything needed and work similarly to the wall tiles. I made a graphic to show how these are cut.
-The 24x24 pixel pieces in the corners are the actual corners (red).
-The 48x24 pixel pieces between the corners are the edges connecting the corners (green).
-The middle 48x48 pixel piece is used to fill the space between the edges (blue) and can be cut in 24x24 pieces.
-The upper-right square contains the inside corners (yellow). 24x24 pixel for each corner.
RPG Maker uses the upper-left corner for the preview in the editor.
On a map it works like this:
And thank you very much! Hope this helps and have fun with the tilesets :3