Step 1. Flare your nostrils and coincidentally look funny.
Step 2. Breath steadily in through your nose, this serves to warm and humidify the air before it enters your lungs. At this point most people might breath upwards, so to speak, raising their shoulders. To breath from your belly, try your best to push your stomach out. This takes a while to get the hang of and can feel unusual, but after plenty of practice it becomes normal.
Step 3. Exhale through your mouth.
Step 4. Repeat.
This technique can be useful out when riding when the level of effort required increases such as in a time trial can help you pace hard riding.