We’ve found that melting chocolate on the stove is easier and more consistent than in the microwave, though both methods work.
Stovetop method —
Put chopped chocolate in a heatproof bowl that fits snugly over a pot of barely simmering water (or put the bowl directly in a wide, shallow skillet of barely simmering water). Stir occasionally until the chocolate is melted and smooth; remove it from the heat. Don’t let water come in contact with the chocolate, and be sure the bowl and spoon or spatula are perfectly dry. Small amounts of water can cause chocolate to "seize," or turn into a stiff, grainy mess. Seized chocolate can be brought back to a smooth state by adding cream, but this can affect your recipe, so it’s often best to start over and set the seized chocolate aside for a recipe that calls for a lot of cream.
Microwave method —
Put chopped chocolate in a microwave-safe bowl and heat it at 50% power for 1 minute; remove and stir. (In a microwave, chocolate doesn’t lose its shape as it melts, so you must stir it to gauge how far along it is.) Return it to the microwave and repeat, stopping to stir every 15-seconds to prevent scorching.