Oh gosh Cheryl.. let me try to explain my version... People do it sooo many different ways... some use cake for the "top" some use fondant wrapped cardboard for the top. I like to use GP and form it around the bottom of the pan that is going to be the bottom of the box... and just let it harden on top... then pull it off the (wrapped and cornstarched) pan and flip it over to dry more on the other side... can take up to 3 days to dry...then after dry I cut a cardboard round/square that just fits inside the lid so the dowell that holds the lid up doesn't go through it. Prop it up at the height you need it so as not to squish your decorations that are in the "box" 
I'm sorry if that isn't a great explanation... 
Here you are -
http://forum.cakecentral.com/cake-decorating-ftopict-215781.html
And welcome to the site! 
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