Depends which icing
If you want to do normal icing or buttercream here are the instructions
These can both cover 12 small fairy/cup cakes or a 20cm cake.
These are instructions from the back of Tate+Lyle Icing sugar and from my own experience
Buttercream Icing
75g of butter
175g Icing Sugar
1 tbsp milk
Put the butter into a bowl then sift in the icing sugar and beat them together until they have started to mix in together. Once this happens add the milk and beat until the texture of the icing is thick and if you tilt the bowl to one side, the icing moves but super slowly. If it moves faster than super slowly, add a bit more icing sugar to get the mixture thickened.
Glacé Icing
225g Icing sugar
2-3 tbsp water
Sift the icing sugar into a bowl and slowly beat in the water to give a smooth icing. Make your you use it straight away
Hope I helped! Good luck!