Using a sharp chef's knife, slice each zucchini lengthwise into very thin slices, as thin as you can. Slice each of those slices into thin, noodle-like strands. Don't worry about it if your slices aren't even.
Alternatively, make your life easy and buy packaged zucchini noodles at the grocery store. I get mine at Whole Foods.
Heat the butter in a large skillet over medium-high heat. When melted and foamy, add the zucchini noodles. Cook, stirring often, until just tender, about 5 minutes. Add the garlic and cook, stirring, 1 more minute. Turn heat off.
To the cooked zucchini noodles in the skillet, add the salt, pepper, pepper flakes and Parmesan. Mix and serve.