Choose a setting for your poem. This can be anywhere and does not have to be communicated to the reader outright. It can be a fictional place or a place from your life, but it is important that you know where it is. If people are interacting with the snow, you should also have a basic understanding of who they are.
Avoid being cliche. So many snow poems start with the line, "Snow is falling," or some variant that implies the same. Don't let yourself be ordinary. Make your poem different from any other. For a list of snow poems for reference, visit http://www.poemhunter.com/poems/snow.
The length of the poem is up to you. Use adjectives in abundance as well as adverbs and metaphors to give your poem more description. These tools give your poem visuals for the reader to latch onto and relate to.
Edit your piece. This can be very tricky and the longest part of writing. Sometimes just reading it backward can give you ideas on how to make it flow better. Keep an open mind while editing. Mixing lines and sentences around can be fun and exciting. Finding the most lyrical way of communicating is what poetry is all about.