You need to ice your cake with either a meringue buttercream (chill it before you cover with fondant) or ganache. You can get sharp corners with these and when chilled they get really firm. Then you need to roll your fondant quite thin, 1/8th inch I believe (correct me if I'm wrong).
The most important thing to remember is whatever your iced cake looks like is what your fondant finish will look like. Any imperfections will show through.
i would definately go with Ganache, You can do it with buttercream, But its a lot lot more faff on than ganache! (ive only ever achieved it once with buttercream, and i think it was more of a fluke than anything )
you can buy special tools, to help create sharp corners, ive never used one so i dont know if they actually work, but it might be something work looking into if thats what your after
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