I've never made a stacked cake before. Would it need some kind of support to stop it collapsing? Also how do I make the cakes look ring shaped instead of disc shaped? I can carve the cakes to roughly the right shape but how do I get the fondant to go round the bottom of the cakes.
You need cards under each cake and dowelling rods to support the weight. If you use cards that are each a bit smalle than the cakes you make you could carve it into the right shape then the fondant should follow the shape of the cake pretty easily. You could leave a tiny bit extra round the edges and then raise the cake up and tuck the extra fondant under the card.
You need cake cards. There are lots of places online you can buy them, or if you have a cake decorating shop anywhere local to you. I'd say you need a bigger difference in the size of your cakes than just 2cm. That would leave just a 1cm difference round the edge, iyswim.