Make sure you take your butter out to soften several hours before.
(Do not try softening it in the microwave, that never works. You end up with slightly melted butter and that will not make good icing at all.)
Cut the butter up into small pieces and beat it with your mixer.
Next, add in about 1/3 of the powdered sugar (amounts in the recipe below) with 1 tablespoon of milk. Starting with your mixer on low beat the mixture well. As the powdered sugar mixes in you can turn up your mixing speed.
When that is beat repeat that twice more so that you have added all the powdered sugar and 3 tbsp of milk.
At this point icing gets a bit personal, as in you have to decide how soft you like it for spreading purposes.
If you want a more spreadable icing you may want to add in more milk a teaspoon at a time until you get it to the right spreadability.
If you want to put it on cupcakes you might want to stop at 3 tablespoons of milk, and may need to add in a bit more icing sugar if it is not stiff enough to stay piled on top of the cupcakes like you want it too.
As I said in the blog post --icing thickness is really up to the individual and more of an art than a science. The best thing to do is add the milk slowly and a little at a time.