Think of a Ganache as an emulsion, just like a mayonnaise. The goal is to mix two components that are not supposed to mix - chocolate,  which is fat, and a liquid - Although the liquid is often fresh cream with some flavor, it can be any liquid. Fruit juices are an obvious alternative. The proportion of chocolate and liquid vary depending on the type of the chocolate you're using - the "lighter" the chocolate, the less liquid needed - and the usage of the Ganache. If the Ganache

Tempering chocolate

Why is this necessary? If you simply melt chocolate and let it cool down on its own, the resulting chocolate will likely: Lack shiny appearance Be soft when broken Rapidly have white color traces appearing This will be due to the cacao molecules not being "aligned" because they will have cooled down gradually. The process of tempering consists in bringing the chocolate to a temperature of 122F or 50C - well above melting temperature of 81F or 27C, then bringing it down to these levels, 80F - 28C and finally

Melting Chocolate

The Micro wave method The goal is to obtain the chocolate in an homogeneous liquid paste and at a temperature not exceeding 50C or 120F. I recommend using a micro wave because the worst ennemy of chocolate is water. Any small amount of water in the chocolate and this will trigger an emulsion and make it more solid and thick and would  turn it into a kind of poor quality Ganache. Although using a double bath works perfectly as well, it presents an obvious risk of introducing water in the choccolate, if only through the steam.