A double boiler is used to heat something delicate, like milk or chocolate, that is at risk of burning if you put it over a direct flame. You can buy a real double boiler, but in my opinion there’s no point when you can so easily make a fake one using other kitchen tools you already own, either a bowl over a pot or two pots.
Bowl and pot
You need a metal or other heatproof bowl that fits on top of a saucepan. The bowl must to be large enough to completely cover the top of the pot so that the steam can’t escape, but not so large that it is at risk of falling off – it shouldn’t extend more than an inch or so beyond the pot.
They should be about the same size. Put the larger/heavier one on the bottom.
How to double boil
Put a couple of cups of water into the (bottom) pot, put the bowl or second pot on top, and add the milk or chocolate or whatever. Turn on the heat and bring the water to a simmer. Stir the milk or chocolate frequently until hot / melted.
Double boiling tips
Don’t fill the water pot more than halfway or you risk the water spilling out over the top.
Turn the heat down to keep the water at a constant simmer, rather than an all-out boil.
Remember that if using the first method, the bowl will be very hot, so be sure to use oven mitts to pick it up.