When you’re installing mouldings that are shorter than the length of the walls you’re installing them on, the moulding ends on the wall, and not in line with the edge of the wall like it should. This can create very sloppy work! This can happen with aprons under windows, chair rails that are being applied over wainscoting (wooden panels that cover the lower parts of the walls of a room). To fix this problem, you can simply set a miter on a small scrap of the same moulding, then glue it into place. Simple! Be sure to hold it in place with tape (masking tape preferably) until the glue dries.
Also recommended to you "Casing Corners the Easy Way".