Here are the basics
Anything in nature that makes air rise will help clouds form. Fronts, mountains, convection, and storms all move air upward. As air rises it cools by expansion, and if it cools below its dew point clouds will form. Clouds are made of tiny droplets (or ice crystals) that form as vapor condenses on particles (pollution, dust, smoke, etc.).
To see a longer version with more details (instructions), CLICK HERE.
To see my video of a student activity, CLICK HERE.
To obtain a student "Cloud in a Bottle" lab activity (handout and answer key), CLICK HERE.
Here's another great video from MinuteEarth - CLICK HERE to see it.