I just made a large buttercream Topsy Turvy with fondant accents, you can see it here, http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1346468
Before I stacked the middle tier onto the bottom tier, I placed the middle tier into the freezer for about 30 minutes to freeze the buttercream solid. This makes it easy to handle without messing up the buttercream while stacking it onto the bottom tier. Freeze the top tier before stacking it on the middle tier too.
Just work quickly because the bc does thaw out fast.
Once you have the tiers stacked the rest is easy! 
another question do you cut the hole to stack like on the demo or just stack it on the next tier I had one tier fall apart when I cut a hole like demo when stacking the next one it fell and took part of the bottom tier with it . I dowel them then put a dowel all the way through all tiers Im so scared now to do the next one and 1 have an order for one in 4 weeks do you take the cake all stacked or put it together at site
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