In a large skillet, heat the butter over medium-high heat.
When the butter stops foaming, add the green beans and salt.
Cook, stirring often, for about 5 minutes.
Add the black pepper, garlic, and crushed red peppers.
Continue to cook, stirring often, for about 5 more minutes, until the beans are tender but not mushy. Serve immediately.
Video
Notes
The most important thing is to avoid overcooking the beans. You want them tender, but they should still have a bite - much like "al dente" when cooking spaghetti. You definitely don't want them mushy or droopy.
Whether you cut the beans in half or leave them whole is up to you. Both work.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Reheat them gently in the microwave at 50% power.