The best way, is to drill (1/32") a hole in the sight blade, for a protrusion (peg) of ivory on the backside of the bead.
This will give a larger bonding surface.
The bead must be made with the peg or protrusion out behind the round 'front'.
Another way is to use high-temp silver solder (brazing silver) and have a rounded puddle on back of the sight blade. Temperature control is vital to achieve this.
Hopefully there are other "better" ways.
I've never done either of these.