If you don't own a set of double-boiler pans, improvise one by nesting a heat-proof glass or metal bowl on top of a saucepan. Glass holds the heat better than a metal one, which conducts the heat faster.
For the top part, use a fairly big wide bowl, not a small deep one -- for instance, if you are melting chocolate, the larger surface area helps it to melt at the same time, preventing it from burning from some being melted and some not.
Be certain that about half of the bowl can be inserted into the saucepan. Most importantly, make sure the water doesn't not touch the bottom of the bowl or top pan. This is especially important because melting chocolate or custard could scorch from the direct heat of the hot water.
You want it to cover the entire top of the pan so steam escaping from boiling water can't get out during simmering and cause seizing when melting chocolate.
Be careful when removing either bowl; they get very hot.