Tie the yarn to one side of the string loop(the lower end where you start twisting the ends together)with a single half-hitch leaveing about a 1" tag end.Lay the tag end against the doubled section and wind the yarn over the tag end and continue down the string untill you get below the doubled section.Insert the end of the yarn between the string bundles and cut leaving about a 1" tag end.String your bow and shoot it a few times,then trim the tag end close to the string.
Hope this helps.....