This is one of the most common breathing exercises that you can do perhaps, anytime and anywhere (even taking breaks in between work). However, it is the most effective if you can do this first thing in the morning on an empty stomach. It requires you to be focused and disciplined while helping to calm your mind.
Here is the set of instructions:
Begin with an easy seated posture and rest your palms on your knees - Sukhasana ("Easy Pose")
Take your right hand and bend your index and middle finger, such that they touch the palm of your hand, slightly close to the bottom of your thumb
Leave your ring and little finger free, and keep your thumb free as well
Now take your right hand close to your nose, and place your right thumb on your right nostril and your ring finger on your left nostril
Gently close your eyes and allow yourself to feel your presence in the room, on the yoga mat
Gently put pressure on your right thumb and block the opening of your right nostril, then inhale through your left nostril
Gently put pressure on your ring finger and block the opening of your left nostril, then exhale through your right nostril
Keeping your ring finger in place, inhale again through your right nostril
Gently release your ring finger and put pressure on your right thumb to block the opening of your right nostril, and exhale through your left nostril
This is considered as 1 round, and you can repeat from Step 5 to continue with the second round
Always start the first inhale through your left nostril, and take deep and long breaths.
You can do this breathing exercise for about 8-10 rounds and try to keep the timing of your breaths consistent, ranging from 5 - 10 seconds for each inhale/exhale. To help you with sitting still, you can play soft music - avoid playing music with a catchy rhythm as it might distract you from your practice. You might struggle to concentrate or perhaps get the left/right sequence mixed up for a start, which is perfectly normal. Be patient and focus on your breath.