Here is how I do it. I made a jig which screws into the bolt body, and has clamping provisions to hold the saddle of the bolt handle firmly against the body. The handle must be located so that its shoulder will contact the receiver way and stop rotation. It must also be located longitudinally so that primary extraction is maximized, while the bolt can be smoothly closed.
My jig being screwed into the bolt body acts as a heat sink. I use high temperature silver solder paste (fluxed silver braze) from Brownells. The bolt and handle surfaces must be clean. No traces of oil. The bolt body and handle are assembled in the jig, with silver solder paste applied in the saddle area. Anti-scale anti-creep paste is applied to the bolt including the threads to protect it and keep the silver from going where it is not wanted. Wet rags are wrapped around the bolt body. I use an oxy-acetylene torch to quickly bring the saddle area up to red heat. The silver will flow and be visible all around the joint. You want to see this - then you know that you have 100% coverage. Remove the heat, let the silver set. Once set I cool the rear of the bolt, handle and jig. Clean up, making sure there is no silver that is going to interfere with bolt operation. Polish, brush, blast, blue, park. as desired for finish. Maybe I've been lucky, but a handle that I've attached has never come off. Knock on wood.
An existing bolt handle is removed by heating the joint to cause the factory induction braze to let go. Use heat paste to protect the bolt body and threads. Wrap body in wet rags.
I hope our member from Fairbanks AK sees this thread, He is very experienced with the job; uses a different method with TIG welding.