Turn your idea into something fantastical. Your myth can be silly or serious, but it should involve something that doesn't happen in the real world. For example, a volcano might erupt because giants underground left the barbecue on. A hero might be forced to learn to care about people after an evil serpent turns his family into trees.
If you're having trouble coming up with a mythological explanation for the topic you chose, write down a list of words that remind you of snow. If you want to explain how snowstorms happen, write down "cold, wet, white, snowman, ice cream, clouds." Maybe snowmen live in the sky and sneeze snow down to earth, or maybe the clouds try to give us ice cream that melts on the way down.