The first example is the ‘Golden Ball’ Christmas cake! (Victoria Beckham eat your heart out!) How to create the look is detailed more precisely in my post, techniques of icing.
The next example is a more traditional style with Santa and the sleigh and the reindeer!
And then there is the Star Cake (#1), a very traditional theme,
And then there is Star Cake (#2) which is a little easier,
And finally, a basic icing nozzle pattern from The ‘Womans Own’ articles to show what the individual tubes can achieve!