it depends on your sketch idea or pic but you would use a 2 layer cake and carve it into the ramps and make skate boards out of fondant check out this link for ideas http://www.coolest-birthday-cakes.com/amazing-cake.html#c3
some diagrams to assist in visualizing
http://www.cakecentral.com/cake-photo_1166372.html
also be sure to search the galleries for even more ideas
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