Seems like an easy thing, right?
Take a deep breath. Close your eyes. Just breathe.
Forget about what you did yesterday, what you forgot to do. What you need to do later. Think about this:
Feel the air moving in through your nose, filling you from head to toe. Exhale. Feel it rise from your toes all the way up and out of your mouth.
Which each breath, focus on the parts of your body that are the most tense. Which each exhale, release that tension.