There really aren't "instructions" anywhere on doing them....just print out alot of photos from Google Images as to what you want to do...and then set them out in front of you when you stack and carve the cake. Most of the time you have to put straws and cardboard halfway up to support the cake, and sometimes people use dowels and a platform attached to a wooden base to support sitting-up "legs".
Most 3D work is a matter of trial and error, and all is very individualized work.
Good luck!
I don't know if this would help you.
http://www.flickr.com/photos/17658856@N03/1841511180/
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