I put a thick bottomed shot glass in a small saucepan with water and put the burner on low. I hit the contents of the shot glass with a heat gun every now and then and try to keep it under 140f, usually more like 100f. I stir stir stir, and take the shot glass out of the pan and stir on the counter some more and put it back in and stir, back and forth. So I sort of regulate the temp like that and try not to leave it in the pan too much. It's usually more about persistence. I'm usually mixing for over 5 minutes.
I know the info is probably around here somewhere but I have had no luck. I want to melt some shatter into ejuice or pg and put it in a tank. Of course I don't want any of it to vaporize while doing it.