How do snow flakes turn into crystals?

Answer 1

Snow flakes *are* crystals of frozen water.

Answer 2

Tiny water droplets that make the clouds cannot freeze without giving away some energy since their mass is so little.
When such a droplet meets e.g. a corn of dust, it freezes. Then, if the air is very humid, other tiny droplets come in contact with it and freeze too, each time creating a different pattern to what will grow up to a snow flake that will eventually fall down when big enough to be attracted by gravitation.