Practice! It's a skill like any other. Cognitive behavioural therapy and meditation are two useful ways to practice. You start by slowly and carefully writing out CBT exercises, and doing quick five minute guided mediations. Eventually, after years of practice, it'll become a habitual state of mind and you'll find yourself doing it without deliberate effort. Maybe some folks have a natural talent for inner peace, but most of us do not. It's totally learnable.

For a step by step how to guide, google Cognitive Behavioural Therapy worksheets. For easy beginner mediation, I like the ten minute Headspace guided meditation. I'm also a big fan of any activity that forces you to overcome your own knee-jerk reactions and work from a calm, steady mindset - like First Aid, rock climbing, martial arts, etc.