First, gather your ingredients.
There are three: icing sugar (I store mine in a tin, it helps me pretend I'm fancy), milk, and butter or margarine (soft but not melty will make things easier on you). Consider these the gold, frankincense, and myrrh of icing. The knife is unnecessary, except to cut the butter/margarine if you forgot to take it out of the fridge earlier and now it's way too hard.
Now all you gotta do is mix these three ingredients until they come out in a good-looking consistency and good-tasting flavour. Here's my system for doing so:
1. Dump a bunch of icing sugar into the bowl. Use a cereal bowl - you'll already end up with way too much icing as it is, no need to psychologically trick yourself into going even bigger.
2. Pour a little bit of milk in. THIS IS THE TRICKIEST PART! Why? Because you will pour too much milk in. I guarantee it. I almost always do. This will eff you over later. Start with what seems like waaaay too little milk and go from there.
3. Mix what you've got together.
4. Add a hunk of butter.
5. Mix that together. If you didn't remember to take the butter out early and it's way too hard then get ready for a workout.
6. Keep adding icing sugar until the consistency is gets nice and thick so it won't run off the edges of the cake.
7. Add a bunch more butter. This helps make the flavour delicious and makes the consistency nice and smooth. Go ahead add a little more. Now more. If you're dieting, what are you doing making icing in the first place? Add more butter. Mix and taste as you go.
8. Want to add flavour? Throw in a splash of vanilla or mint-juice for deliciousness. If you want to make it chocolate then start adding cocoa powder into the mix and include it in your mix/taste procedure from now on.
9. Keep mixing and adding butter and icing sugar until it tastes and looks good. Keep mixing any lumps out. Keep mixing. If your arm is getting tired, sit down, put something on TV, and keep mixing. It's your own darn fault for not taking the butter out of the fridge earlier.
10. Soon it will be of a delicious consistency and flavour. You are done. You most likely have WAY more icing than you actually need. Cover the remainder with some kind of plastic wrap and store it in the fridge for days when you need a delicious treat. Don't forget though, that this is icing made of real-food ingredients without preservatives, so it will go bad eventually. You have been warned.
Disclaimer: It will not look like store bought icing! Vanilla icing will be off-white, especially if you use real vanilla flavouring. Chocolate icing will be a chalky brown. You want perfectly white or coffee-brown icing? Then find out what chemicals and dyes the manufacturers put in their icing to make it look that way. It's fake and it doesn't make it taste any better. The best thing you can do to make your icing delicious is keep adding butter and icing sugar. For reals.