If you read the installation instructions, they tell you to use a file to contour the parts to fit the curves of the shower. A bit harder than just cutting it straight, but that's what's required to get a good, water-tight fit.
Personally, if I had to do it over again, I wouldn't buy a Kohler shower door; I'm not thrilled with the fit and finish of mine and it's only about 6-years old. I might break down and buy a new one, and trash the thing. Inertia has held me back (and the money, too!). Eventually, it'll look crappy enough that I'll break down and do it.