I spread the icing over the top of the cake first, then the sides. Trying to spread it the icing evenly and ensuring no yellow of the marzipan is peeping through is the hardest bit.
I like to go for a snowstorm effect for my Christmas Cakes (because it's easiest!). I just use the flat of the spatula and tap the cake, the icing sticks an gets pulled into peaks as you pull away. See snowy?!
Then just stick your decorations onto the wet royal icing. Easy.