you need:
a long serrated knife
an icing knife (or a large butter knife or big spoon)
a nice serving plate for the finished cake

1. the cake MUST be cooled completely.

2.  for two layers: cut the cake in half shortways, so you have 2
    squares.  these will be two layers.  one layer will have to be
    trimmed so that it makes a flat surface for the other layer.  fit
    the two halves together with icing inbetween the layers.

3. begin with the crumbing.  (i don't know what you really call this)
   take a small amount of icing and put it into a separate small bowl.
   apply a THIN layer of this icing to the whole cake.  the point is
   to seal in the crumbs so they don't mess up your final
   presentation.  you will get crumbs in this icing, and in the little bowl.

4. now apply a nice smooth beautiful layer of the non-crummy icing all around.