I make soap from time to time with my mom. Nothing for shaving use but just for showering.
The point of a double boiler is not to let the soap come in direct contact with heat. If it does, such as a pan on stove, microwave it will dry out the soap. The glycerin will break down, fragrance will be stripped and sometimes the color diminishes. Using a double boiler is to allow the water to come into direct contact with the heat and using the steam to slowly melt soaps. The highest temp I use is about medium on the stove. And, stir frequently... sometimes those soaps won't melt properly and need to be stirred. Just as Glen said, just enough to melt it and pour. You really don't need much.
If you have a transparent mug and would like the soap to be a little more transparent, try tossing it in the freezer until hard. Be careful, expansion and contraction could crack your mug.
This same method works great for chocolate and strawberries. ;)
This is just what I know from playing around with soaps. I am not a professional soap maker but what I have mentioned does work.