If you click on Paris on the left http://www.subwaynavigator.com/subway_site/php/home.php?langue=eng and type St-Lazare and Gare du Nord without the quotes you just may decide to walk - it's approximately 5 minutes on the metro.

If you decide to go by metro anyway, make sure you know the name of the last stop in the direction you're traveling as that is what it will say on the front of your train.

Cheers, Linda