How do snowflakes know how to form six (or twelve) close to identical arms?

Mother Nature gives them a blueprint

Then once they have the blueprint; what making them follow it?

Snowflakes are crystals, which grow based on the original frozen molecule. At the original freezing point, a six sided prism, with hexagonal top and bottom. is formed, and then as the crystal grows it warms and freezes to grow branches off each face of the prism.

During the crystallization process, the water molecules align themselves to maximize attractive forces and minimize repulsive ones. As a result, the water molecules arrange themselves in predetermined spaces and in a specific arrangement.

This process is much like tiling a floor in accordance with a specific pattern: once the pattern is chosen and the first tiles are placed, then all the other tiles must go in predetermined spaces in order to maintain the pattern of symmetry. Water molecules simply arrange themselves to fit the spaces and maintain symmetry; in this way, the different arms of the snowflake are formed.

One can manipulate crystals in ceramic glazes to an extent.
Round,flower or ax shaped, large, small, few or many can be attained by tweaking ingredients in a glaze along with firing schedules.
I’m currently doing experiments with this and it’s a lot of fun.
Here’s some examples of crystalline pottery
There will be a test :)

I love the last snowflake picture. It has little ones arranged as a tail, like a kite. I think it tried to stay in flight.

The photographs are awesome!