<div id="gbqfb" style="z-index: 1000;"></div>
The snowyfog-mod.js (you can just save it with right click from here) goes into the same directory as your html file.
Update: You need to include the Google JS api to have the "google"-environment available. Please add your API-key which you can get in the Google API-Console. The script tag should be added in the head section:
Here is the example page with minimal requirements: Let it snow!
When opening your page it will slowly go foggy. After a while a button called "Defrost" will appear and all the fog will be removed after clicking. Snowflakes will continue to fall. Try it out.