Makeshift Double Boiler
If you don't have a double boiler, you can create your own using a metal bowl and a saucepan. The bowl should partially (not completely) fit into the saucepan. Fill the saucepan with enough water so that when the bowl rests in the saucepan the water does not touch the bottom of the bowl. Bring the water to a simmer. Add the ingredients to the bowl and place it in the saucepan.