Remove all decorations and toppers (including the edible ones!) and get a decent serrated knife and cutting board.
Make a straight cut across the curved edge of the cake, you can use the width of the knife to work out how big the slices should be. Use the blunt side of the knife to indent the cake to indicate where you should cut.
Cut straight across the cake, and continue making cuts inwards of the cake. The number of cuts you make will depend on the size of the cake.
Lay the slab that you have just cut down on the cutting board.
Cut the slab vertically into four portions, and cut once vertically across for small portion slices.
I've made a helpful video of how to cut a cake, watch it here.