Start in a comfortable seated position with minimal distractions around you.
Step 1 - deep inhale through your nose
Step 2 - hold your breath for 2 seconds
Step 3 - long exhale through your mouth
Focus on your breath.
It is natural for your mind to wander, so when it does you need only come back to your breath.
Pay particular attention to the long slow breath out.
As you become more comfortable with this practice, start to imagine the breath as it flows in and out of your lungs.