No expert here, but I have done a few of these.
Well, I didn't find the instructions all that complete.
So here goes and I hope it helps.
First of all, very important, use a good dense cake. I foolishly used a marble cake for my first one and even doctored (cake mix) it was not dense enough to support the tremendous weight of the fondant and icing and the cakes.
I used three inch layers and put two of these together for each tier of the cake. So I froze them and then cut the top off on a slant as described in the instructions. Then I sculpted them while frozen. Then I did the stiff buttercream dams both in between the two layers of each cake individually and also where the slanted top was put on, with a buttercream icing. So are you still with me, haha? This amounts to each separate stacked tier of your cake actually being like a three layer cake with filling in two places, where the two layers are joined and also in the top layer where the slanted top is joined.
So if you are doing the fondant thing, well first, once the cake is defrosted (while it is still covered with plastic wrap), you are going to get your fondant ready and rolled out. Now you are going to ice the cake with a thin coat of buttercream, I did this even in the hole that you create for the next cake to fit into.
Now two different approaches when covering with the fondant. I have done both. Either you cover the cake with the fondant and then cut out the hole out of the fondant on the top where the next layer of cake is going to sit into or you press the fondant into this hole as you cover the cake. Most folks leave this hole on top, only covered with buttercream, not fondant, some people fit in a piece of fondant afterwards and some cover the whole cake and press in the indentation where the next cake will sit.
What did I find worked best? Actually loosely fitting the fondant over the whole cake and then cutting the indentation part out, then smoothing out the fondant over the whole cake and then when it was perfect, re-inserting the cut piece of fondant into the indentation.
Well, I did it with both, 2, two inch layers and 2, three inch layers. SOme folks do it with one 3 inch layer and this works well. The original topsy turvy whimsical cake was actually done with a 4 inch deep pan, which are next to impossible to get anymore, generally because they were hard to bake with.
The dowels are very important and double boarding the cake boards that each tier sits on is also advantageous, trust me on this one. The centre dowel through all of the layers is really important if this cake must be moved.
I can tell you that the marble one I did that was a partial disaster, well, here is what happened. I was fortunate that a good cake buddy, Labrat, had posted a disaster she had and so I was on the look out and watching mine very carefully after it was constructed.
So I took a good look at it every few minutes, now I had all of these cakes dowelled well and I also had a centre dowel through them and had double boarded them. I thought that the cake was looking a bit more tilted and so I watched it and some cracks developed in the middle layer. Now this cake was 3, double layer of 3 inches each, so 6 inch high cakes, the sizes were 5. 6 and 8 inch rounds.
So once I saw the cracks in the middle layer, I removed the top two layers, and there was cracking at all of the areas where the centre dowels were. So this tells you that the cake itself was not strong or dense enough to support all of the weight. (this was exactly the same thing that had happened to Labrat)
So what I did was patch up the middle layer and put it back on top of the bottom cake and then transport the top cake separately. This was a trial cake that I had made for a family friend, so it wasn't for a customer.
Don't know if this is just a trial for you, or for something special, but this is one cake that I would most definitely do a trial run on. I also would not transport even a two layer whimsical cake without a centre support.
Hope that helps a bit,
Hugs Squirrelly Cakes