I always make more than I think I will use, because it’s easy enough to refrigerate/freeze the remainder. I usually go with a 2# chocolate, 8 oz butter in a bowl, and 2# heavy cream brought to the boil (so it climbs the sides, but you have to watch carefully otherwise it makes a mess!) and pour that over the chocolate. Shake the bowl a bit to ensure all the cream finds it’s way to the bottom of the bowl, wait 3-5 minutes then stir gently. It helps if the butter is cut up, room temp is ok but if you use it straight from the fridge, it is definitely best to cut it up. Ganache will lose it’s sheen as it stands on a cold cake, but you can replace 1 or 2 ounces of the cream with corn syrup (put this in with the cream) for more shine. I let the ganache firm up over several hours (depends on the ambient temp in the kitchen) or even overnight so I can spread it on; or wait a few minutes and use it as a glaze to pour over a cake.