Or, if you just want to make a firm ganache, it's just a matter of incorporating air and cooling it down.
The ganache recipe I use is simple but very versatile.
Equal parts chocolate and heavy cream, with a little unsalted butter to taste. For example:
1 lb. dark chocolate chips
16 oz. heavy cream
4 Tbs. unsalted butter
Scald the heavy cream, then pour into the chocolate. Allow the cream to melt the chocolate.
(If you want to use this as a pourable ganache, also add a little corn syrup -- 2 Tbsp. if you're using the amounts above, then whisk until well incorporated, but you don't whisk in too much air or you'll get bubbles. Then you can pour it over your crumb-coated cake, etc.)
For a spreadable/pipeable ganache, whisk with an electric mixer, preferably a KA because it can take a long time. I was required to this by hand when I was in school, but it was very painful
. The key here is to incorporate a LOT of air -- otherwise, it will be rock solid and unspreadable. You may need to periodically place the bowl over an ice bath or place in the fridge to firm up slightly.
If it gets too firm to use, re-melt it over a hot water bath and repeat the mixing process. I put leftover ganache in the freezer, then make it usuable again this way.