Is it hard to find the RER E at the Gare du Nord? Is it within the station itself or do you have to go outside by the metro station? On a map it looks like the metro is across the street from the train station.
You will see signs directing you to the underground tunnel to the RER E station.
this link is very helpful - photo essay of the transfer - scroll down for Metro directions.
https://www.about-paris.com/gare-du-nord-to-montparnasse.html