So basically all you need to do is decorate since we have already baked the cake.

So you slice your cake into 2 halves then add your Nutella and some M&M into it then cover with the other half

Next your cover the body of the cake with Nutella(it doesn't matter if is not smooth) then you start arranging your wafer round the cake and when this is done you simply pour your M&M on top and that is it....