Water vapor is the gas form of water and is usually invisible unless temperature or changes in pressure cause it to condense. When condensation occurs, the invisible water vapor changes from a gas to tiny particles of liquid water suspended in the air. This results in clouds or the visible steam that can come out of your mouth. Your lungs are full of moist air, so making water vapor come out of your mouth is easily done by either using cold temperature or high pressure.
Method 1: Cold Air Method
Stand in a location with very a low temperature. This can be outside in the winter or indoors in front of a freezer.
Take a deep breath and exhale slowly through your mouth. You should see a visible cloud generated by your mouth.
For a large cloud, exhale slowly with a wider mouth. Breathing out quickly will create a fainter cloud. Experiment to create the effect you prefer.
To see a longer lasting effect of the water vapor, breathe onto a piece of glass in the cold location. The cloud will condense and remain on the glass for a longer period of time than in the air.
Method 2: Mouth Pressure
This method can be done anywhere in any temperature. Begin by taking a deep breath.
Fill your cheeks with air while continuing to breathe through your nose.
Put pressure on the air in your mouth without allowing any air to be released. This will allow the moisture in your mouth to evaporate into water vapor under high pressure.
After a few seconds, quickly release the air from your mouth. Saying a short word or sound like "pah" will aid in the quick release.
You should be able to see a small cloud emit from your mouth due to the excess water vapor being exposed to normal pressure again.