Hey Bill,
I use the unserved portion of an old endless loop string for my tie on nocks. About 6 or 8 inches so you can still hold on to it good, you just snug it down enough that it holds and doesn't slip.
Sorry, I don't know how to describe it any better but what I do is make a series (3 or 4) of over hand knots only knotting on one side of the bow string. I've tried knotting on both sides & it seems easier to "screw" up & down the string if I only knot it on one side. On the last knot, I make sure I tie it so that the last knot can be a square knot. I finish with just a pin prick dab of glue on the knot, duco/fletching glue, just whatever I have handy.
I have tried making two series of knots, one on top of the other and didn't like it too well. I settled on this method after winging it one day while getting tired of adjusting a brass knocking point. I'm not sure if others do it this way or not.
Shane