Let it snow is a javascript that makes snow in any HTML block.
Just add the following line somewhere in your HTML code:
<script src="letitsnow.js" type="text/javascript"></script>
Then assign some element, in which you want snow, to the class letitsnow, like this:
<div class="letitsnow">
This should work out of the box, for most simple CSS-layouts. If not, read more here.