I've created a few makeshift double-boiler setups over the years but am always unsure of the proper setup. I dug around online but nothing very thorough (or reliable-seeming) came up. My Better Homes and Gardens cookbook basically just says the double boiler needs to be heat-safe and should not touch the water below. Is that all there is to it? Should it / can it form a seal with the pot below? Should the water be simmering or boiling?