For those who want to choose shape of snow flakes I suggest to download instructions how to choose snowflake shape with more than 10 snowflake images like two snowflakes below.
Following above link you will find step by step instruction how to get custom images of snowflakes on your site. It is accomplished with code like this:
Following this link will also find URL's for [PathToSnowfallJsFile] and [PathToImageFile].
For guide how to implement basic snowing effect on blogger (without custom shapes) follow the link.