Cut the mortise

The tapered slot that the new neck will fit into extends up to the second peg hole. This gives an adequate gluing surface for a strong joint.  There are several styles for cutting this joint; this is the French style and is the simplest, least invasive and is adequately strong. The English style calls for a taper that ends in a butt-joint at its small end. It is trickier to cut and gives no advantage.