To make the icing
Combine the icing sugar and lemon juice in a bowl. Mix these together until a runny icing has formed. If it's too thick, add more lemon juice, a teaspoon at a time. If it's too runny, add a tea spoon of icing sugar at a time to thicken it.
Wait until the doughnuts have completely cooled before decorating them. Once iced, top with sprinkles, or however you like!