I use 200g butter to 1kg icing sugar, and 125ml of room temp milk, as a base. This is enough to ice 24 cupcakes, for example.
Beat room temp butter for 3 minutes. Add half the icing sugar and the milk. Beat for 3 minutes. Add the remaining icing sugar and beat for another 3 mins.
For lemon, I add the zest and juice of 1 or 2 lemons - you will need to reduce the milk.
For strawberry, reduce the milk and add strawberry jam as needed
(with or without pips, as you like it).
For chocolate, I add 90g of cocoa powder and 200ml milk in total. Add the cocoa with the icing sugar.
Because of the low fat to sugar ratio, this icing crusts. If you don't want it to crust, use more butter, less liquid.