First off, what do I mean by "Be Peaceful"? I mean a state of mind where there is no agitation, no stress, no anxiety… something analogous to the surface of a placid lake.
And by "No Matter What", I mean even in these example circumstances:
- Someone is yelling and insulting you
- You are confronted by an armed robber
- Someone very close to you dies
- Someone does something that goes against one of your important principles
- A natural disaster is in progress
The first step is to realize that you are different from your thoughts. This may not happen immediately, but will definitely happen with continual practice. 10 minutes a day is all it takes.
The upshot of realizing you are separate from your thoughts is being able to quietly watch them.
So let’s consider the thoughts that will be running through your mind in one of the above circumstances.
Let’s take the first one (believe me, almost every person on the planet has been there!)… let’s say you and your partner are having a heated argument, and your partner is slamming you with f’s, b’s and s’s.
Instead of screaming back impetuously, separate yourself from your thoughts and watch them. You’ll probably see thoughts like, "Who does s/he think s/he is?!!" or "s/he’s not giving me the respect I’m due" or "I want to slap this person hard"…
But just keep watching them. This prevents you from getting angry and enables you to keep thinking with a clear head.
Watch what your mind suggests you do about the situation. Your train of thought will start off being very similar to what it would be if you were not identifying with the basic you, but at a certain point will change for the better. This is because as the basic you, i.e. as a peaceful witness to all your thoughts, you will personally not feel insulted (such feelings may occur, but you will be calmly watching them and not identifying with them), and your train of thought will not continue on the initial agitated or angry track.
Once your mind decides on what your response should be, you (i.e. the basic you) make the body act accordingly by doing or saying what the mind decided.
Take note that the key principle here is to watch your thoughts during the event and let the mind decide what it wants to do, then act accordingly. If you lose your cool, you’ve blown it 
The takeaways:
- Spend 10 minutes a day to realize that you are separate from your thoughts
- Use this separation to keep a cool head during events that are typically stressful.
Good luck, this method works for me and it will for you too!
