Take an arbitrary surface. Let's use an orange peel with a cut in it as an example. Spread it out and flatten it onto a surface. Any places that resist too much (i.e. have too much curviness to flatten well), make a tear from the edge to that point to relax some of the tension. Repeat until it's adequately flat.
Remember this has to work on *any* surface (Platemail, anyone? Orc skin, perhaps? Topographic map of Nepal?) and you see why this can be rather difficult.
It gets even harder when you want to make tears in places that won't be very noticable.