Is to butt one of the moldings up to the corner. Then take the piece that is going to mate to it and cut a 45 degree miter on it so the saw cut is showing towards the face of the baseboard. Then take a coping saw and cut away the face of the cut that is showing, following the contour of the cut. Butt the new piece up against the piece that is already in the corner, and you will have a perfect match. The beauty of this method is that it is not sensitive to the corner angle, and there is no gap to open up when the baseboards are nailed into place.