I'd say buy Sharon Sugarshack's DVD "Boxes and Bows" but you don't have enough time. I have it but have only watched how to do the first one which is a square cake with a lid on it. For that cake, you add tylose powder to fondant so that it will dry. Roll it out and cut out a piece slightly larger than the finished cake will be (you will cut it again once the cake is finished) and cut four pieces for the sides (slightly longer than the side of the cake). Dry flat for 3-4 days. Carefully flip pieces every day or so so that the bottom can dry. Once cake is iced, carefully measure the finished dimensions of the top and make a template out of paper. Then using the template cut the piece of fondant that has dried. You don't want it to be so hard that it shatters or cracks when you cut it. The drying time depends on how much tylose you add and your weather/fondant/etc. Apply the tope to your cake using melted chocolate or royal icing as the glue. Then apply each side using the "glue" after measuring each side. Make sure you measure the side you are working on AFTER the previous sides fondant has been placed so that the seams match up.
Thais is the best way I can describe it in words. Truly, a picture is worth a thousand words.
I've made a lot of gift cakes in my gallery using the technique shown here:
http://www.make-fabulous-cakes.com/gift-box-cake-tutorial.html
didi5 thanx for the great link! I have always wondered how to make that cake using only buttercream. I had no idea it was that easy 
there are a few dfferent ways to approach a lid
dried panels
fondant overlay
gumpaste one propped on side
faux lid....
showers above did a good job describing how i do my paneled one
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