The Double Boiler Method
If you don't own a double boiler you can fashion one out of a stainless steel or glass bowl set over a saucepan. Be certain that about half of the bowl can be inserted into the saucepan, and that the bowl covers the entire top of the pan.
Whether you use a real double boiler or a makeshift version, you only need about an inch of water. Most importantly, make sure the water doesn't not touch the bottom of the bowl or top pan. This will help keep the chocolate from scorching. I recommend melting coarsely chopped chocolate over medium heat while stirring frequently with a rubber spatula until the chocolate is completely melted and smooth. Melting it too quickly over high heat may render scorched, inedible chocolate. Avoid introducing any moisture to the melting or melted chocolate; it may seize (the chocolate stiffens into a coagulated mass, suitable only for the garbage disposal).
How long will it take?
2 to 3 ounces will melt in 2 1/2 to 3 minutes
3 to 4 ounces will melt in 3 1/2 to 4 minutes
5 to 6 ounces will melt in 4 1/2 to 5 minutes
7 to 8 ounces will melt in 5 1/2 to 6 minutes
9 to 16 ounces will melt in 6 1/2 to 8 minutes
The Microwave Oven Method
The microwave is a very efficient piece of equipment for melting chocolate. It's quick -- about twice as fast as a double boiler -- and it is relatively foolproof as long as you don't microwave for longer than necessary. (Just as with a double boiler, too much heat and the chocolate will scorch.) Microwave coarsely chopped chocolate in a glass bowl. After removing the chocolate from the microwave oven, use a rubber spatula to stir it until smooth.
How long will it take?
I always use the medium power setting. Times may vary, depending on the model, wattage and power setting available on your microwave oven.
1 to 3 ounces will melt in approximately 1 1/2 minutes
4 to 8 ounces will melt in approximately 2 to 2 1/2 minutes
9 to 16 ounces will melt in approximately 2 1/2 to 3 minutes
Melted chocolate should stay melted for up to an hour, depending on the ambient temperature in your kitchen. If your kitchen is cool, keep the melted chocolate over warm water until ready to use, unless the recipe requires the chocolate to be chilled before using.
Melting Chocolate with Other Ingredients
Several of my recipes require melting chopped chocolate with other ingredients, such as butter and cream. As when melting chocolate all by itself, a modicum of attention and care is recommended, especially with regard to heat. It's always best to heat chocolate slowly rather than quickly, and over low rather than high heat.
Also be aware that the combination of ingredients actually accelerates the heating and subsequent melting, so it may take less time to melt chocolate with other ingredients than by itself. Keep a close eye.