It is traditional to moisten the outside of the cake with some melted apricot jam and then cover it with marzipan. Then you (slightly) moisten the marzipan with brandy/sherry/water and apply the fondant.
If you are after a smooth finish, it really helps to smoosh some marzipan into any dents or divots after jamming but before covering with marzipan, so that you have as smooth a starting surface as possible. Use your fondant smoothers on the marzipan just as you would on fondant and get the edge as sharp as you want it.
Then just decorate as you would a normal cake.
Hope this helps and you have fun with this cake!