Weigh out the amount of chocolate you need based on the ratio chart below. Chop in small even pieces.
Measure the cream into a saucepan based on the ratio chart below.
Heat the cream over medium-low. DO NOT boil. The cream is ready when it begins to scald (simmer).
Turn off the flame and remove the cream from the stove.
Add the chocolate, stirring gently to distribute it through the cream. Melt. Continue to stir. At first it might look broken – keep stirring until it comes together.
Cool as described below.

How to cool ganache:

To pour ganache over a cake or pastry, it'll need to be viscous enough to flow but thickened enough to stay on the product (pourable, add corn syrup to recipe).
To whip ganache for frosting or for layer cake filling, cool until it's thick, but still soft, and then beat with mixer until fluffy and has lightened in color, about 1 or 2 minutes.
To use ganache for truffles, you may need to set the pan in the refrigerator so ganache cools. Stir frequently to cool evenly. As the chocolate begins to stiffen, stir it more frequently – it'll go from soft to very hard quite suddenly. (If this happens, soften the ganache over gently simmering water, stirring until the right consistency.)

More about ganache:

Choose your chocolate by percent. Taste the chocolate before making your frosting. Chocolate that is 70 percent and above will be less sweet.
Recipe proportions are based on weight. For example, a 1:1 ratio means 4 ounces chocolate to 4 ounces cream. These ratios depend on how thick/thin the end result will be:

-For layer cake filling and thick glaze: 1:1, equal parts chocolate and cream.
-For chocolate truffles: 2:1, 2 parts chocolate to 1 part cream.
-For soft icing and pourable glaze: 1:2, 1 part chocolate to 2 parts cream. (Add 1 to 2 tablespoons organic corn syrup in pourable glaze.)

The easiest way to work with ratios is to measure both the cream and the chocolate by weight. If you don't want to weigh your cream, remember that 1 cup of liquid is 8 ounces.