Below is a cool infographic on how to properly measure snow from the National Oceanic Atmospheric Administration (NOAA).
Since snow doesn't always fall straight to the ground from the clouds, you need to be careful to not measure in drifts that farm near buildings or other objects. Also take multiple measurements to insure the best reading!
This is definitely something fun to do with the kids or any weather enthusiast and if done correctly you can report your results to your local National Weather service office or even your local tv meteorologist who will be certainly looking for snow measurements on social media (Twitter, Facebook). Take a pic of your ruler in the snow and send it with the average measurement to your local meteorologist and it may end up on tv!
Here's a measurement I took in early 2014. We got just about 6 inches of snow!