Measure the length of the wall.
Mark the needed length on a piece of quarter round molding.
Slide the quarter round molding into a manual miter box with the square corner sitting in one corner of the box. Hold the trim in place with one hand while you cut the molding to length. Remove the molding from the miter box.
Hold the quarter round molding on a firm surface with the end of the molding sitting three to four inches past the end of the surface.
Align a coping saw blade with the area the miter cut meets the front---rounded---face of the trim. Tilt the coping saw on a 30-degree angle to the trim face. Cut along the front of the trim to remove the wood from behind the cut.
Install a non-mitered piece of trim into a corner.
Slide the mitered end of the quarter round trim toward the installed piece of trim. Examine the fit to the corner. Trim excess wood from behind the cut, if needed, to create a tight corner seam.