Find a pot that is wide enough to hold a metal or glass bowl nesting in the opening, with the bottom suspended a few inches above the bottom of the pan. You want the bowl to be large enough to rise up and beyond the top of the pot so you can place and remove it easily.
Fill the pan with water, but not high enough to touch the bottom of the bowl. Rest the bowl with the ingredients (chopped chocolate, egg yolks, whatever you are cooking) in the top of the pan, again making sure the bottom does not touch the water.
Bring the water to a simmer (not really a boil, which is usually too hot for this method). Stir until whatever you are cooking has melted or reached the desired consistency. Remove the bowl from the double boiler, watching that the steam doesn’t billow out and burn you. Remove the pot from the heat.
Notes
Make sure to use a heatproof bowl: metal or glass are good choices.
Don't let the bottom of the bowl touch the water in the pot below.
When removing the bowl from the pot, pay attention to the steam that will be released, which can burn.