The same way as a normal cake, but you could always cut the "high" sides a little narrower so that people get the same amount of cake as the pieces on the low sides.
It's important that the carved sides are equal all the way around the cake. If one side is carved in deeper than the other, then the heavier top will begin to lean and you could end up with a slipping top torte.
They don't need much carving as it's the slant in the top that lends mostly to overall effect.
I don't cut the hole for the flat spot as in the tutorial here, I just stack each normally using dowels and piping gel to hold them in place. But with this method I've found that you have to cut an angle form both the top and bottom of the top tiers (not the base tier) so that it looks like a wedge. Otherwise the top tiers will stick out to the side too much.
Try googling topsy-turvy cakes. I read an excellent post with a picturial on how to do it. I got the link from CC. Try our search first. It was very helpful. I didn't save it. I am a visual person.
I hope that helps.
There are a couple different methods - I like the one where you cut the cake at an angle and then rotate it 180 degress so it now creates a crooked tier. Just offset each one and then (IMHO) dowel the center.
If you PM me your e-mail address I'll e-mail you my instructions. I took pictures of a lot of the steps involved too and I'll send those as well. 
chikie - is your question for serving people? Or for construction?
I thought it was for serving it, not for construction!
Ooooh.. DUH! 
I usually just cut it like any other cake.. a little thinner in the taller parts and a little thicker in the shorter parts.
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