I use one block of UK butter and 1 box of icing sugar (I think it's the 1kg one but will double check for your and post back here).
Allow butter to come to room temp and mix icing sugar into it in small batches (careful, it will release a cloud of sugar dust if you do this too aggressively at first!). I use a hand mixer to do this.
To this base, you can add lemon, vanilla, cocoa, cream cheese or any other flavor you like.