First, you need some water vapor to find a dust particle and form a cloud around it.After the water at the top of the cloud starts to freeze, let the rising air tear away some water droplets and bring to the top of the cloud. Then let the heavy ice fall towards the base.
Wait until the electricity potential between the two clouds or the earth and your cloud reaches about 10,000 volt per centimeter.
Let the air become ionized along a narrow path. Lightning should flash, and thunder should boom. You have created a thunderstorm!