Plan ahead and set out the butter to get it to room temperature. If you are careful you can microwave cold butter a little at a time to soften. But do not melt.
Put the butter & the powdered sugar in an electric mixer.
Start mixing slowly and add vanilla.
Add the milk a little at a time to get the frosting to the consistency you need for your cake. You want it to spread easily.
If you are piping a border or flowers add less milk.